Could be caused by a server reboot that I did around that time.
This is what might have happened:
1) You opened a page
2) A HTTP session is started automatically
3) Server reboot
4) after 25 minutes after the page load in step 1, the page will automatically perform a client-sided ajax call in order to keep the HTTP session alive. But because the server was rebooted, the session didn't exist anymore, and thus the error pages were shown.
The same thing can happen when a PC is put on stand-by while keeping the page in the browser.
So if it was caused by the reboot, than it shouldn't happen much. Stateless pages would prevent this problem, but with the cost of spam.