[see the last paragraph for the TL;DR summary]
I use Auth_remoteuser for the *.opensuse.org wikis and hit a problem with the cookies used by Auth_remoteuser.
Trying to save any changes on el.opensuse.org or pl.opensuse.org always errored out:
Sorry! We could not process your edit due to a loss of session data. You might have been logged out. Please verify that you're still logged in and try again. [...]"
Editing the other wikis worked.
The reason for the session problem was $wgCookieDomain = 'opensuse.org' instead of $LANG.opensuse.org
el.opensuse.org includes MediaWiki:Common.css from en.opensuse.org and gets the english session cookies this way. Since the CSS is loaded after the HTML and the latest cookie wins, the next request sent the english cookies to the el wiki.
The solution was obviously to restrict $wgCookieDomain to $LANG.opensuse.org.
The better solution would be to somehow include the wiki name (maybe using the database name?) in the cookie name - instead of MediaWiki_Extensions_Auth_remoteuser_AuthRemoteuserSessionProviderUserID it could be something like MediaWiki_Extensions_Auth_remoteuser_AuthRemoteuserSessionProvider_wiki_en_UserID. This would avoid cookie conflicts if $wgCookieDomain covers more than one wiki.