I'd re-check all the power and assistance settings are correct and nothing is "zeroed" .. Though that would be unusual if it was working and now isn't.
You could check the torque sensor. Not sure if Besst gives a live feed ? The "Go" app in android (not go+) gives a live torque sensor...
Forgot to ask ..
Does walk mode work and have you tried connecting an android phone with the "Go" app installed to see if you get a torque sensor reading from the motor ?
Lots of useful crap (or not ...) in here , it's more a data thread than a chat thread, there's a main e-mythique chat thread.
Part way down the first post discusses the lock ring tool ..
https://www.emtbforums.com/threads/vitus-e-mythique-tech-docs.35325/
Didn't you try this before ??
Click on the three dots.
Then a drop down appears. Click on the camera with the plus in the middle.
Select the video file. It will only show files it can upload.
I don't think it does the "set to zero" everytime. I've heard of a few others besides me where it has happened though.
I think I put all the default settings in the tech docs thread if you do get the problem.
I think mudguards are a must. There is a thread - generally good advice on any EMTB.
https://www.emtbforums.com/threads/e-mythique-mud-guards-the-other-vitus-and-maybe-most-none-fixed-rear-triangle-bikes.35852/
I'm still problem free 6 months on, so it's not a certainty that you will have...