The trainer that old is made for windows 7 and below.
The things you can try is to run the trainer as admin.
check your AV / Windows defender and make sure nothing is blocking the trainer.
Also
www.laptopmag.com/articles/set-compatibility-mode-windows-10
Unfortunately, I have tried all that already with compatibility etc. What I also don't understand is why the trainer "ignores" the offline key and still attempts to authenticiate. Any thoughts on this?
As a sidenote: I wonder if there would be any way to generally remove the authentification requirement from the old trainers? Piracy prob won't really be an issue with them anymore....
Unplug your ethernet cable from the back of the computer, Key only works if there is no internet signal at all.