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). -
Using Ftp rename plugins folder to say plugins-hold and create a blank plugins folder then try logging in
If you can log in you will see all plugins deactivated go back in ftp delete the blank plugins folder and rename plugins-hold back to plugins.
Activate and update one plugin at time to see which one is causing the problem
switching to the Twenty Ten theme to rule out any theme-specific problems? -
re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress?