I am trying to add one release from a RG to a series. (It is an old series and new releases are not described as being part of the series.)
Incorrect entity type for part of series relationship at lib/MusicBrainz/Server/Data/Relationship.pm line 565, <$fh> line 1.
Well, it's a release group series, so you need to link a release group to it. If this should be a release series instead, you'll need to change the type first (since it's empty right now, you should be able to just edit the type)