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 Ten 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 the wp-admin and wp-includes folders from a fresh download of WordPress.
– increasing the memory available to PHP:
https://www.ads-software.com/support/topic/253495#post-1017842
