Problem:
Unable to log into a Drupal site. Logging in with valid credentials would fail without any visible error in the UI.
Solution:
Repair the sessions table.
When I exported the database using Sequel Pro, I got the following error:
“Table ‘./d6_eiper/sessions’ is marked as crashed and should be repaired’. I was able to do this with Sequel Pro, but you could also use phpMyAdmin.
Update 4/6/2012: More tables starting crashing in the same way and we were able to repair them as well. The reason for these crashes is still a mystery. We restarted the web server and haven’t seen any crashes since.
Aaron Cole WP.com 