We have been having the same issue as this with the built in app on a Samsung KS.
The last time, I stumbled upon hitting back and finding the app springing to life, but thought it might have been a fluke. The issue was back today, though saw the post above. The back button worked again!
Here’s an update for everyone, I was having the issue for months with the app, it wouldn’t recognise my username or password and I tried everything.
So today I reset my Samsung TV to Factory settings and everything is working, the TV has automatically updated all my apps and now TV is working perfectly. Definitely try it, it’s in the Settings section, Support, then Reset.
Hope it works for you!