Hi, I've been trying to install newly bought witcher from GOG, but after downloading all the files there comes installation error 1.5 (BŁĄD INSTALACJI 1.5 in my polish lang version). The game is running fine from shortcut in start menu, but it seems that patch 1.5 cannot be installed. What can I do to fix it?

64bit windows 7 professional, core i5-6600k, integrated GPU (waiting for Polaris)