Feb 20 2020
False alert, sorry! It worked on a second try. Probably my cookies were still broken from a previous login. Phew!
BTW, I am running MediaWiki 1.34.0.
Arrrgh! I just upgraded from PHP 7.4.1 to 7.4.3 – and that bug is back again!
Apr 6 2017
Okay, so I fixed the wikiuser grants manually, and now it works. Thanks! Still a mystery how the grants could get broken …
Apr 5 2017
Well, wikiuser always had proper permissions since I installed Mediawiki back in 2006 and the database underwent many updates since. So I am wondering how it could get broken by upgrading to 1.28.0 …