We have encountered this error this week.

Moodle 2.6.1+
Chrome Version 33.0.1750.154 m
Firefox 28.0
IE (9, old version intentionally installed)

Replicating:
1) Access course with large amount/number of files in the course, and preferably a huge number of files in the Moodle instance in other courses.
2) Edit a section (type of content is not a problem)
3) Select 'Insert Image' -> 'Find and Upload an Image'
4) Select 'Server Files'. Display files as Tree (IMPORTANT)
5) Before 'server files' loads, select another item such as 'Recent Files' and then 'Upload a File' - clicking either.
6) Depending on the session time out, this will throw the "Session Timeout" error when the timeout is reached.
7) The window is then "frozen", with no ability to select anything on the page - such as close the upload window.
8) Reloading the window then also returns the "Session Timeout" error and the system is not usable until the window is closed and session cookie is lost (as happens when the window closes).

Work around:
Update the users selections in mdl_user_preferences 'filepicker_recentrepository" to 3, and "filepicker_recentviewmode" to 1

What is expected:
Session does not remain locked while AJAX calls are running during the load of 'Server Files' page; so that if you browse away from the 'Server Files' page prior it does not cause cascading 'Session Timeout' errors.

What Actually Happens:
Session is locked while AJAX calls are executing, browsing away from 'Server Files' before it loads causes an error stating that the session is locked, then further locking errors in the application on page refresh. Requires the user to close the browser window and start a new session (or clear existing session cookie for that site)