Have you tried:
- deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).
- switching to the Twenty Eleven theme to rule out any theme-specific problems.
- resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
- re-uploading all files & folders - except the wp-content folder - from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones.
- running the upgrade manually via wp-admin/upgrade.php