I was unable to add new forums to the Tapatalk app on my Samsung Galaxy S3, the older forums I had been using for some time were fine just no new ones could be added so I deleted the app and re-installed it, I then had no forum access even to the old forums. one of the sites I visited where this exact same problem was being discussed suggested deleting the app and installing an older app from an apk file, I did that and I could then log in to all my forums, I then did an update to the latest version and it is still working and all good so far.
Pete