I was able to fix the exact same problem with my Roku by contacting HBO Max using the following method I found on the internet:
"The phone number for HBO Max customer service is (855) 942-6669. This isn't actually listed anywhere on HBO Max's website. However, that number is listed on Xfinity's, and I was able to confirm it by calling it myself.
Looking for technical support, all I had to do was dial that number and press "2" when I hit the first set of branching options in the phone tree."
After describing my problem, the person helping me disconnected and reconnected Max on their end and then walked me through reconnecting to Max on my Roku. It worked! Good luck to the rest of you.