Thought I would add I am running into this problem as well. The error occurs when I attempt to go to my website from the subdomain address. If I go to https://www.badcharms.com/slblog all works well. When I try to use https://slblog.badcharms.com , I receive the add_cap fatal error.
I’ll wait to see opinions on how this adds to or causes the issue before I make Jonathan’s edits.
UPDATE: I made Jonathan’s edits after all due to issues being able to log back into the site’s dashboard. Still will be following this with interest, however.
