Successive failures since activating MultiSite Plugin Manager.

I resolved an issue in the capabilities.php file after getting a fatal error, but after that I got the error when accessing any of my domains (ie. <domain>/wp-admin):

You do not have sufficient permissions to access this page.

The front-end of the site is working fine, but the back-end login fails.

I’ve looked in quite a few forums, etc., but there’s no clear way to even diagnose the problem.

Where do I start?

Thanks … Michael

PS. I really want to use Multisite Plugin Manager, so hopefully the solution isn’t to avoid using that plugin.