On German Language, where I am a moderator, I wanted to swap the tag synonym determiner → determiners (and then delete the plural variant) in the tag-synonyms interface. When I try to do that, I consistently get a pop-up:
An error occurred; please try again later
I can swap other synonyms, so this is not due to the tag database being out of order or similar. However, what does separate this synonym from others is that its creator is “anon”, i.e., the creator of the synonym has since then deleted their account. I tested this with another tag created by “anon” and encountered the same problem.
With another tag that suffered from the same problem (I didn’t check for the creator), I could delete the synonym and then rename the tag, so there is a work-around. It’s just direct swapping that doesn’t work.