So, I will post this since it might me helpful to others (with the same issue) in the future:
My nano X died after some days of use. I did not notice it right away because I only used it for 2 days 3 months ago and then picked it up again yesterday.
I suspected a faulty LiPo, so I swaped it with the one from an older Winning remote. Again no luck, it still didn’t charge and the voltage of the new battery kept lowering. After that I tried resoldering: still no luck…
It turned out that if the remote was off, it would not charge the battery. So it olny charges when it is ON.
Im quite sure this should not be this way, if yes let me know.
Result: Some hours wasted + One dead Lipo at 1,2V (Original one) …
Im not sure what enertion charges for this remote, but not even having a under voltage protection is too much.