The reason due to which you are getting UDB error message, could either be some data based issue with your AT&T email account or you might be entering incorrect details while trying to sign in.
If you think that the details that you are using while trying to sign in on your home page, are correct and still getting the same UDB error, I would suggest you to contact AT&T Tier 2 support at 1-877-722-3755 (open 24/7), they will check your AT&T email account details and help you with this issue accordingly.
When you call in to speak to an agent, ask to be transferred to our second level of support.
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*I am an AT&T employee and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.