Problem upgrading from 7.22 I'm trying to update from Drupal 7.22 to the current (7.26) version. I keep getting the following error on the Configuration page after updating.

Fatal error: Class 'DatabaseTasks_mysql' not found in /server/path/stuff/here/domainname.com/html/modules/system/system.install on line 188

I have tried upgrading incrementally 7.22 -> 7.23
I've tried upgrading directly to 7.26
I've tried uploading files via FTP, following the standard upgrade instructions, and I've tried upgrading with drush. I always get the same error. (though the line number does change depending on which version I'm updating to.)

Does anyone have any suggestions?
I was experiencing this problem on two different test servers (different Debian and PHP versions) using data imported from our production server. All of the cache tables were emptied first, and I did try reinstalling Drupal using various methods. Couldn't find a solution anywhere.

What finally fixed it for me was emptying the registry and registry_file tables, then running update.php.  (Update didn't find any database changes, but it reset the registry - clearing the cache may have worked as well.)

Hope this helps someone - this problem was driving me crazy for two days.