You could try deleting (after backing up, if your cautious) all the files in your wp-admin/ directory and re-uploading freshly downloaded copies.
Few people have made customizations to files in that directory, but occassionally people do (sometimes an older plugin requires such modifications, for example). Hopefully you are not one of those people; hopefully you can delete at will. If you have made customizations, you’ll have to back up your files before you delete them and then redo the customizations.
EDIT: To be honest, though, I have no idea if this is the source of your problem.