cross post from dav3.net post on this subject
The other day I upgraded my wordpress sites using the backend upgrade and I lost all permissions to access ANY admin area. Any guest could use the site as normal, there were no problems there, but I couldn’t login. It took a few days searching on the web (without luck) and eventually, I jumped back into cpanel, and phpmyadmin and started digging.
The solution was a lot simpler than what I had envisioned.
During the update process, one of the plugins caused havoc in the backend, and when trying to display the dashboard, the plugin was breaking privileges. First I had to work out, which plugin it was, and if in fact if it was a plugin that was the cause. As it turned out, it was, and this is how I found it:
1. created a new folder called “old_plugins” *
2. moved all the plugins from the “plugins” folder to the “old_plugins folder” *
3. logged in – sweet – I was allowed and all appears good
4. visit the plugins page (you will get a warning that your plugins have been deactivated since they can’t be found.
5. put the plugins back into their original directory *
6. reactivate each plugin until you find the faulty one. For me it was a google reader plugin for the dashboard, which I never used anyway – so it’s now gone!
Once you find the faulty plugin, you’ll get the permission error again, simply go back to your ftp or file manager (I just used cpanel) and move the faulty plugin out…
* You could probably have just renamed the plugins folder, visited the plugins page, re-renamed the plugins folder, and gone through the reactivation process that way, this is just the way I did it.