If I edit an empty board description, and click "Save description", I get the message Previous revision identifier is missing
Reproduced at https://www.mediawiki.org/wiki/Talk:Sandbox
If I blank an existing description, that does Not cause the bug to appear.
However https://www.mediawiki.org/wiki/Talk:Sandbox used to have a description until 2 days ago, so that's odd...
Ah, I replicated part of the edits from Talk:Sandbox at https://www.mediawiki.org/wiki/User_talk:Quiddity_%28WMF%29 by adding [[Category:Flow|*]] and then deleting it, and now that page is also suffering the bug.