"This social network account is already used by another user."

Hello!

I had problems setting up Facebook login originally several months ago. This was, because Facebook didn't permit me to use any admin account, but I had to use my personal name and send an image of my ID card. I got it working after some swearing.

However now a new user discovered, that Facebook login no longer works. It turned out, that my Facebook account doesn't exist. It seems, that Facebook has deleted it because of inactivity (I don't use Facebook in any other way).

I created a new Facebook account using another personal email address I have. I got the Facebook log-in running and tested it by logging in into my site with this account. It worked, a new local account with my name (with visible username John Smith) was created, but my original (non-admin) local account was renamed (its visible username became John Smith2). Realising this I deleted the Facebook originated user in the WP backend, which restored my local account's visible username.

Somehow my original account got linked to a Facebook account instead of a Google one. I unlinked it, but I can't relink it to Google. I just get an error "This social network account is already used by another user" as if something remains of the deleted user. How can I solve this mess?