I got the game in a sale in a few days ago and spend now a few days researching this topic, as I had this from the start.
Sadly I was not able to fix it but I would like to sumarise some methods:
Starting Gog and the game in Adminmode = didnt worked
trying to start the game in compatibility mode = not possible, there seems a problem with a file, I cant find out what
checking game files = didnt worked
tried the unofficial patch, just to be sure = dont solve that bug.
Found a rather new topic from steam with someone who managed to fix it, apparently there is something wrong with a certificate. Reinstalling this certificate solved the problem for some, I tried that too = didnt worked
The only other thing Ive found was, that it worked if one has a gog friend and both play at the same time, it will work with that friend. That I wasnt able to try.
If I understand it correct, that only influences how the own vassel gets rented out. So the vassal wont get quest and beast knowledge, wont bring back any recieved presents and dont get payed.
As I can still rent other peoples vassals, but I am not sure, if they can recieve gifts, I stopped giving departing vassals gifts.
What about the other online feature, the undead dragon - does someone know, if that works at least correctly?
Vassal bug is annoying, but not as impactfull, but if the dragon fight wont work properly, there is no use in doing that fight.