S-a-y

If your location is correct, maybe, really just maybe, did you use non-latin characters in your username?

If so, try to change to only latin bases characters, sometimes this causes problems. Also sometimes certain special symbols might cause problems as well (like @! or similar and talking only about username/charactername NOT password ;) )

I do not have non-latin characters, however I used czech localization for game. I switched back to english and it works (but I cannot customize that ugly fonts).

I owned the game purchased in steam. Claimed it in GOG and I had the same problem with the registration. The game does not detect the purchase/registration.
But not tried again, maybe now it works. Just uninstalled and keep the steam version for now.

Bought it on GOG and created a Runic account that lets me login just fine on their website. Logging in in the game though results in Account not associated with purchase. I am using the username of my Runic account too so I don't know what could be going wrong. Any help would be really appreciated.

Okay, created a new account and the difference between this and the old one is that the username didn't contain any hyphens in the new one. Logged in successfully with this account.