I also have this problem. I didn't sign up to the beta program as a developer, but Apple support directed me to here to ask my question as they deleted my post off the regular forum for mentioning the word beta. Telephone support suggested I just keep waiting it out and 1 of the beta updates might fix the issue but I have been unable to login for months, and have done several beta updates since. Hoping someone knows how to get round this. Support advised me against unenrolling and then trying to re-enroll.