Thread:Rappy/@comment-6152051-20150606050249/@comment--20150606052317

Near the bottom at "Known issues". It says "We're currently looking into an account session issue that may cause edits or uploads to fail (with an error message like 'your session expired'). If you're being affected by this, please let us know any details via Special:Contact. Thanks!".

Something to do with editing/uploading seems to be buggy for some users.