We have an issue with a contributor from the French Wikimedia, Kvardek du.
When they try to save an article on fr.wikipedia.org or other *.wikipedia.org, a //session_fail_preview// error message (es.). Behavior persists after cookie clearing. Behavior doesn't persist in private browser mode, so it looks browser-session related.
A debug request with log enabled has been done on mwdebug1002 @ 2016-12-25T18:13:43 - 18:13:44. It's available on logstash. No error is logged when the user tries to edit, but when they visited en.wikipedia just afterwards, two kind of error type occur:
* a [[ https://logstash.wikimedia.org/app/kibana#/doc/logstash-*/logstash-2016.12.25/mediawiki/?id=AVk3MPW4EOj-wvmp3eFm | session error: ]] when some JS code wanted a JSON response but got a PHP Notice: A session had already been started ignoring session_start() (possibily because of the next?)
* **a [[ https://logstash.wikimedia.org/app/kibana#/doc/logstash-*/logstash-2016.12.25/mediawiki/?id=AVk3MPvpEOj-wvmp3eID | TitleBlacklist error: ]] PHP Warning: Compilation failed: missing ) at offset 63 @ /srv/mediawiki/php-1.29.0-wmf.6/extensions/TitleBlacklist/TitleBlacklist.list.php line 43**
In VE mode, there is no issue.
On a side note, on Flow, it's possible to write topic, but when trying to delete them a ''Invalid CSRF token'' message is received, then the topic is deleted.
Finally, could be related to T151770 as this contributor also expected this issue on Firefox.