The adjusted instructions for the new docker container with Solr9 works just fine.
The improved output also helps us to understand what is going on:
Fri May 30 10:08:39 AM UTC 2025: Done fetching MusicBrainz Solr backup archives.
Solr is running on http://localhost:8983
load-backup-archives: Fri May 30 10:08:40 AM UTC 2025: Load MusicBrainz Solr collection (1/15): 'annotation'...
load-backup-archives: Fri May 30 10:08:40 AM UTC 2025: Delete the pre-existing backup 'annotation'...
load-backup-archives: Fri May 30 10:08:40 AM UTC 2025: Extract the backup archive 'annotation.tar.zst' (339M)...
load-backup-archives: Fri May 30 10:08:41 AM UTC 2025: Restore the collection 'annotation' from extracted backup...
load-backup-archives: Fri May 30 10:08:45 AM UTC 2025: Delete the no-longer-needed extracted backup 'annotation'...
load-backup-archives: Fri May 30 10:08:45 AM UTC 2025: Done loading MusicBrainz Solr collection 'annotation'.
load-backup-archives: Fri May 30 10:08:45 AM UTC 2025: Load MusicBrainz Solr collection (2/15): 'area'...
load-backup-archives: Fri May 30 10:08:45 AM UTC 2025: Delete the pre-existing backup 'area'...
load-backup-archives: Fri May 30 10:08:45 AM UTC 2025: Extract the backup archive 'area.tar.zst' (38M)...
load-backup-archives: Fri May 30 10:08:45 AM UTC 2025: Restore the collection 'area' from extracted backup...
load-backup-archives: Fri May 30 10:08:47 AM UTC 2025: Delete the no-longer-needed extracted backup 'area'...
load-backup-archives: Fri May 30 10:08:47 AM UTC 2025: Done loading MusicBrainz Solr collection 'area'.
load-backup-archives: Fri May 30 10:08:47 AM UTC 2025: Load MusicBrainz Solr collection (3/15): 'artist'...
load-backup-archives: Fri May 30 10:08:47 AM UTC 2025: Delete the pre-existing backup 'artist'...
load-backup-archives: Fri May 30 10:08:47 AM UTC 2025: Extract the backup archive 'artist.tar.zst' (912M)...
load-backup-archives: Fri May 30 10:08:51 AM UTC 2025: Restore the collection 'artist' from extracted backup...
load-backup-archives: Fri May 30 10:08:55 AM UTC 2025: Delete the no-longer-needed extracted backup 'artist'...
load-backup-archives: Fri May 30 10:08:55 AM UTC 2025: Done loading MusicBrainz Solr collection 'artist'.
load-backup-archives: Fri May 30 10:08:55 AM UTC 2025: Load MusicBrainz Solr collection (4/15): 'cdstub'...
load-backup-archives: Fri May 30 10:08:55 AM UTC 2025: Delete the pre-existing backup 'cdstub'...
load-backup-archives: Fri May 30 10:08:55 AM UTC 2025: Extract the backup archive 'cdstub.tar.zst' (90M)...
load-backup-archives: Fri May 30 10:08:56 AM UTC 2025: Restore the collection 'cdstub' from extracted backup...
load-backup-archives: Fri May 30 10:08:58 AM UTC 2025: Delete the no-longer-needed extracted backup 'cdstub'...
load-backup-archives: Fri May 30 10:08:58 AM UTC 2025: Done loading MusicBrainz Solr collection 'cdstub'.
load-backup-archives: Fri May 30 10:08:58 AM UTC 2025: Load MusicBrainz Solr collection (5/15): 'event'...
load-backup-archives: Fri May 30 10:08:58 AM UTC 2025: Delete the pre-existing backup 'event'...
load-backup-archives: Fri May 30 10:08:58 AM UTC 2025: Extract the backup archive 'event.tar.zst' (85M)...
load-backup-archives: Fri May 30 10:08:59 AM UTC 2025: Restore the collection 'event' from extracted backup...
load-backup-archives: Fri May 30 10:08:59 AM UTC 2025: Delete the no-longer-needed extracted backup 'event'...
load-backup-archives: Fri May 30 10:08:59 AM UTC 2025: Done loading MusicBrainz Solr collection 'event'.
load-backup-archives: Fri May 30 10:08:59 AM UTC 2025: Load MusicBrainz Solr collection (6/15): 'instrument'...
load-backup-archives: Fri May 30 10:08:59 AM UTC 2025: Delete the pre-existing backup 'instrument'...
load-backup-archives: Fri May 30 10:08:59 AM UTC 2025: Extract the backup archive 'instrument.tar.zst' (1.5M)...
load-backup-archives: Fri May 30 10:08:59 AM UTC 2025: Restore the collection 'instrument' from extracted backup...
load-backup-archives: Fri May 30 10:09:00 AM UTC 2025: Delete the no-longer-needed extracted backup 'instrument'...
load-backup-archives: Fri May 30 10:09:00 AM UTC 2025: Done loading MusicBrainz Solr collection 'instrument'.
load-backup-archives: Fri May 30 10:09:00 AM UTC 2025: Load MusicBrainz Solr collection (7/15): 'label'...
load-backup-archives: Fri May 30 10:09:00 AM UTC 2025: Delete the pre-existing backup 'label'...
load-backup-archives: Fri May 30 10:09:00 AM UTC 2025: Extract the backup archive 'label.tar.zst' (87M)...
load-backup-archives: Fri May 30 10:09:00 AM UTC 2025: Restore the collection 'label' from extracted backup...
load-backup-archives: Fri May 30 10:09:01 AM UTC 2025: Delete the no-longer-needed extracted backup 'label'...
load-backup-archives: Fri May 30 10:09:01 AM UTC 2025: Done loading MusicBrainz Solr collection 'label'.
load-backup-archives: Fri May 30 10:09:01 AM UTC 2025: Load MusicBrainz Solr collection (8/15): 'place'...
load-backup-archives: Fri May 30 10:09:01 AM UTC 2025: Delete the pre-existing backup 'place'...
load-backup-archives: Fri May 30 10:09:01 AM UTC 2025: Extract the backup archive 'place.tar.zst' (26M)...
load-backup-archives: Fri May 30 10:09:01 AM UTC 2025: Restore the collection 'place' from extracted backup...
load-backup-archives: Fri May 30 10:09:02 AM UTC 2025: Delete the no-longer-needed extracted backup 'place'...
load-backup-archives: Fri May 30 10:09:02 AM UTC 2025: Done loading MusicBrainz Solr collection 'place'.
load-backup-archives: Fri May 30 10:09:02 AM UTC 2025: Load MusicBrainz Solr collection (9/15): 'recording'...
load-backup-archives: Fri May 30 10:09:02 AM UTC 2025: Delete the pre-existing backup 'recording'...
load-backup-archives: Fri May 30 10:09:02 AM UTC 2025: Extract the backup archive 'recording.tar.zst' (42G)...
load-backup-archives: Fri May 30 10:14:40 AM UTC 2025: Restore the collection 'recording' from extracted backup...
load-backup-archives: Fri May 30 10:17:40 AM UTC 2025: Delete the no-longer-needed extracted backup 'recording'...
load-backup-archives: Fri May 30 10:17:41 AM UTC 2025: Done loading MusicBrainz Solr collection 'recording'.
load-backup-archives: Fri May 30 10:17:41 AM UTC 2025: Load MusicBrainz Solr collection (10/15): 'release-group'...
load-backup-archives: Fri May 30 10:17:41 AM UTC 2025: Delete the pre-existing backup 'release-group'...
load-backup-archives: Fri May 30 10:17:41 AM UTC 2025: Extract the backup archive 'release-group.tar.zst' (2.7G)...
load-backup-archives: Fri May 30 10:18:21 AM UTC 2025: Restore the collection 'release-group' from extracted backup...
load-backup-archives: Fri May 30 10:18:32 AM UTC 2025: Delete the no-longer-needed extracted backup 'release-group'...
load-backup-archives: Fri May 30 10:18:33 AM UTC 2025: Done loading MusicBrainz Solr collection 'release-group'.
load-backup-archives: Fri May 30 10:18:33 AM UTC 2025: Load MusicBrainz Solr collection (11/15): 'release'...
load-backup-archives: Fri May 30 10:18:33 AM UTC 2025: Delete the pre-existing backup 'release'...
load-backup-archives: Fri May 30 10:18:33 AM UTC 2025: Extract the backup archive 'release.tar.zst' (3.6G)...
load-backup-archives: Fri May 30 10:18:59 AM UTC 2025: Restore the collection 'release' from extracted backup...
load-backup-archives: Fri May 30 10:19:06 AM UTC 2025: Delete the no-longer-needed extracted backup 'release'...
load-backup-archives: Fri May 30 10:19:06 AM UTC 2025: Done loading MusicBrainz Solr collection 'release'.
load-backup-archives: Fri May 30 10:19:06 AM UTC 2025: Load MusicBrainz Solr collection (12/15): 'series'...
load-backup-archives: Fri May 30 10:19:06 AM UTC 2025: Delete the pre-existing backup 'series'...
load-backup-archives: Fri May 30 10:19:06 AM UTC 2025: Extract the backup archive 'series.tar.zst' (7.7M)...
load-backup-archives: Fri May 30 10:19:06 AM UTC 2025: Restore the collection 'series' from extracted backup...
load-backup-archives: Fri May 30 10:19:10 AM UTC 2025: Delete the no-longer-needed extracted backup 'series'...
load-backup-archives: Fri May 30 10:19:10 AM UTC 2025: Done loading MusicBrainz Solr collection 'series'.
load-backup-archives: Fri May 30 10:19:10 AM UTC 2025: Load MusicBrainz Solr collection (13/15): 'tag'...
load-backup-archives: Fri May 30 10:19:10 AM UTC 2025: Delete the pre-existing backup 'tag'...
load-backup-archives: Fri May 30 10:19:10 AM UTC 2025: Extract the backup archive 'tag.tar.zst' (23M)...
load-backup-archives: Fri May 30 10:19:10 AM UTC 2025: Restore the collection 'tag' from extracted backup...
load-backup-archives: Fri May 30 10:19:11 AM UTC 2025: Delete the no-longer-needed extracted backup 'tag'...
load-backup-archives: Fri May 30 10:19:11 AM UTC 2025: Done loading MusicBrainz Solr collection 'tag'.
load-backup-archives: Fri May 30 10:19:11 AM UTC 2025: Load MusicBrainz Solr collection (14/15): 'url'...
load-backup-archives: Fri May 30 10:19:11 AM UTC 2025: Delete the pre-existing backup 'url'...
load-backup-archives: Fri May 30 10:19:11 AM UTC 2025: Extract the backup archive 'url.tar.zst' (4.7G)...
load-backup-archives: Fri May 30 10:19:43 AM UTC 2025: Restore the collection 'url' from extracted backup...
load-backup-archives: Fri May 30 10:19:51 AM UTC 2025: Delete the no-longer-needed extracted backup 'url'...
load-backup-archives: Fri May 30 10:19:51 AM UTC 2025: Done loading MusicBrainz Solr collection 'url'.
load-backup-archives: Fri May 30 10:19:51 AM UTC 2025: Load MusicBrainz Solr collection (15/15): 'work'...
load-backup-archives: Fri May 30 10:19:51 AM UTC 2025: Delete the pre-existing backup 'work'...
load-backup-archives: Fri May 30 10:19:51 AM UTC 2025: Extract the backup archive 'work.tar.zst' (1.5G)...
load-backup-archives: Fri May 30 10:20:05 AM UTC 2025: Restore the collection 'work' from extracted backup...
load-backup-archives: Fri May 30 10:20:07 AM UTC 2025: Delete the no-longer-needed extracted backup 'work'...
load-backup-archives: Fri May 30 10:20:07 AM UTC 2025: Done loading MusicBrainz Solr collection 'work'.
load-backup-archives: Fri May 30 10:20:07 AM UTC 2025: Done loading all the MusicBrainz Solr collections.
I have just set up a new Ubuntu virtual machine with v-2025-05-29.0-solr9 and everything works as expected.
Thanks to @yvanzo and all involved people for this newest docker container update.