If the series is https://musicbrainz.org/series/df84a498-843a-4c5f-97a0-a107fa03109e (I searched for the name visible in the screenshot; but really, please link to things next time), it’s never going to work – that is a release group series, to which you can add release groups, but not individual releases.
(Aside: The “subseries” relationship is going in the wrong direction.)
Seems like @chirlu solved why you’re getting an error, but the “server is being naughty” error is probably not the most helpful error to describe what’s going on.
I think this often happens when people add a series from the relationship editor, because that defaults to a release group series. I have made a ticket for that.
Thanks everyone for your replies. That was the problem indeed. I changed the series type form release group to release (and the subseries relationship direction too). After that, the edit worked fine.
It is true that the error message is not informative; it is actually misleading as it makes the user think the error does not come from their side.
Hopefully the linked tickets will be looked into some time soon.