Since another member just pm'ed me about the same issue, I thought I'd briefly post an update on my situation.
I got in touch with exertis, but of course their advice was just to do a full factory reset (back to Windows 8.1 as I had an original Linx 8 and upgraded to Windows 10).
I didn't expect this to help, but tried it out with an iso off the forum anyway because I had no better idea. It didn't solve the problem, but led to the minor improvement that it seems to be charging directly when I switch it off now (before I had to press Power + Volume down for about 10sec to get it to charge, no idea why).
They also sent me a USB stick to do a full factory reset - I honestly don't expect this to have a different result than the factory reset I already did, but will try it out once the USB reaches me. If/when it doesn't work, at least I can get back to them and tell them that the problem persists, and maybe convince them to either offer me a better solution or a replacement.
I don't think there's a Windows software problem actually, I rather suspect it's something in the hardware, or at least some sort of BIOS/UEFI setting that's gone mad.
If anybody else has a working solution that would be highly appreciated.