I downloaded the game two days earlier and have no problems playing it on Win 8.1 x64. However when playing rev. 2.4 before upgrading from Win 7 x64 I had a similar bug as the firewall (Comodo) popped up in the background to ask for permissions. On other games I just switched to the Desktop, gave permissions and brought the game back to front, but UFOAI crashed. Maybe this helps.