Site repeatedly breaks user session and prompts for log in

I have to log in several times while navigating through the L10N wiki. Sometimes I submit translations only to discover that my session expired (after less than 5 minutes). Other times I get logged out just by navigating from one page to another. Editig pages is nearly impossible, due to the same problem. I wasn't able to identify any logic that could explain the occurrences, and other sections of the XWiki.org site work fine. The server logs should have something to say...

Perhaps I'm using my XWiki.org account to access the L10N wiki (don't know for sure), although the wiki's start page warns that it is not possible. If that is causing the problem, I think the L10N wiki should not have allowed me to log in.