My email is already taken, but is not true


I hope this is the right place to getting this kind of help.
I couldn’t find any other place anyway.

I’m trying to add my private email address to my existing GitLab account (which is using another email address), but the system says that my email is already taken.

Ok, I don’t remember ever creating an account with this email address, but I try to recover my password.
I get a recovery link, I set a new password and I’m welcomed with the TOS and an option to continue with the creation of the account or to cancel the action.

This confirms the account with my private email address does not exist, and yet, GitLab says that it’s already taken.

How can I get out from this loop?