Thanks @Everyone for your tips and help - Honkong thinks I should send it to them, I sent them videos with all the conditions, they have it on their radar. Apparently there are models that no longer recognise the batteries and crash... which mine has now also started to do on the USB-C... Well then...

You have probably already done it, but loaded the latest firmware on it? as I said, it was the case with me once where it was flashing red and after hours where I left it, it had not loaded at all. I noticed that it was extremely hard in the lower area until it engaged properly, a Mü remained outside. Tried it with a bit more pressure and then it was really in. Good luck.

no, never seen it. My problem is that the camera stops recording by itself after an hour...

yes, once. make sure that the battery is really in and that the small shutter slides are properly engaged, i.e. absolutely no red to be seen. if you go into the water with red then the camera is ruined.