Like you I was forever being prompted to create a new account no matter which browser and device I was using. Frustrating. I thought the problem affected only me.
I eventually found that there was a mismatch between my Ubuntu SSO email address and my Discourse email address. Obviously that was something that didn’t affect the previous Discourse release but does now.
So I changed my Ubuntu SSO email address to match that in Discourse and logged in successfully.
As I hadn’t posted with my paulw2u1 account I changed the Ubuntu SSO email address back, logged in to the duplicate account and deleted it. Then I changed the email address back again and continued with my original paulw2u account.
My account preferences show a SAML email address which was the email address that I had changed in Discourse but it’s not the one that I have set to use in Ubuntu SSO.
I hope that helps you restore your account and that I’ve explained in sufficient detail what you need to do.