I’m still having this issue on my discourse instance when creating new topics, every now and then:
If I then remove most of the tags, the creation succeeds.
After that I can edit the title/tags and add the missing tags again, this time without error message.
This issue is present since month/years. I cannot find a pattern, maybe it occurs when trying to create new tags while writing the initial posting, but I’m not quite sure.
Update 1: I’m the website’s administrator and writing the new posting with this account, so I strongly assume this is not a permissions issue per se.