"Sorry! We could not process your data due to a loss of session data"

MediaWiki users have apparently seen this message fairly often, based upon its prevalence -- intentional or otherwise -- on web sites. Less often appearing is commentary on cause.

In at least some of the cases, this is actually a PHP-related problem rather than a MediaWiki problem per se. Usually it's because the php.ini file doesn't have a sensible and writeable directory for the session_save_path that is available to the webserver. Use phpinfo() to learn which php.ini is in use, correct the directory to one that's writeable and accessible to the browser, then restart the web server - usually Apache (not the computer).

This will appear at errorprocessing.com at a later date.