Whenever I try to start the game, a pop-up appears asking for a password.

I neither had previous installations of the game installed nor virtual drives/iso-readers; the laptop still is nearly the same as-delivered.
When I gave up and re-installed the game from the original disc, no such message appeared; thus it has to be something coming from the Gog-port.

[1]: There also was an old topic for the same error, which sadly has gone unanswered for nearly five years: forum/age_of_wonders_series/game_demands_a_password/?search=password [I can't post links...]

May it have anything to do with a PBEM password? Any reason why it may think you're trying to play multiplayer?
Also, is it installed somewhere OTHER than in Program Files? I hear that putting especially older games there tends to cause issues a fair number of times (wouldn't know, I never had a game even on the system partition).

Thanks for the reply. It may have been the location then, I have all my games on a second partition on a second harddrive. After re-installing through Gog Galaxy I could play it, it only affected my "regular" install.