Can't log into my Xfinity account; it is linked to another account, I think

Years ago I created an Xfinity account (it may have been Comcast at the time).  I more recently tried to shop for Xfinity internet and created another account, probably with the same service address.  I think I linked the two accounts.  Now when I try to login the (new) account doesn't work.  After I enter my name and password it takes me back to the home screen "xfinity.com/auth".  If I click on the account icon there, the first option is "sign in", which has the same result if I click on it a second time.

If, instead of clicking on "sign in" I click on "account and identity", it takes me to a page that shows a progress icon (three dots in a circle) for a second, then briefly flashes "something unexpected happened" with some more text below that I can't read.  It then goes back to the progress icon and repeats the process over and over.

I would like to switch my internet provider to Xfinity when my current plan expires in April, but I am afraid to do so if I can't create a working Xfinity account.