It took me a while, but I finally got my various Tapatalks working again.
One version told me that I had to create a Tapatalk account to login to Mobileread, but that's not true. (I don't want to give Tapatalk the login credentials for all of my forums.)
In all cases, I had to delete the old Mobileread forum entry from the Tapatalk app (I closed the app to be safe) and re-add it. The biggest problem was that searching seems to be broken on older versions of the app. I was finally able to add Mobileread by searching for the full URL
https://www.mobileread.com/forums/ .
Hope that helps somebody.