I have similar problems on iOS device.
i am unable to login to Samsung galaxy watch app on my iPhone using Samsung credentials which works well if login through web browser.
I have the same issues with the sports watch update connecting to an android T85 os 7.1.2
My watch will connect to another Samsung phone with no problems on an old s3 phone and an s7 edge. Problem S3 can't do Samsung pay! S7 is damaged. Either my android is obselete or the Samsung experience is back running.
I've been having the same problem for the for the past few days. I have tried numerous of times uninstalling and reinstalling the app and I've tried resetting my watch. I've tried clearing the cache and nothing seems to fix the problem. my phone is a Note 8