It is an error which drive me crazy couple of times. It's easy to fix - but hard to detect why happening. I didn't see any good explanation which fits to each single situation I needed to handle it. There are couple of popular theories on what causes the "jtablesession::Store Failed - DB function failed with error" type of errors, but sincerely I can not confirm any of them:
- Flooding (Check your http access logs for access IP’s, and your PHP error logs). Solution: Try dropping the time out down temporally to a lower value, to see if the sessions clear or not.
- SEF If your site has a lot of visitors table jos_session can accumulate substantial overhead. And if you use default SEF, some 404 pages can make some bots to enter into loop, and those bots gives you a huge session overhead.
- Site statistics from within Joomla. Enabling statistics through your site configuration just adds extra MySQL queries and increases server loads.