Hi all, Happy new year.
After 4 hours with the Apple support they could not help me resolve the issue.
What I did after that and worked is:
Unpair Apple Watch
Set up as new Apple Watch.
30 minutes later everything was up and running
Fitness + works like a charm both on my iPhone and Apple TV.
I highly suggest you do that without wasting any time finding another solution.
You don't even loose any health data. The only thing you'll have to do is re-do your notifications for the native apps and personalise your watch faces.
Hope this helps :)