I was always launching CK2 from Steam using the play button, but on 13th or 14th Feb I launched it directly using its shortcut in the windows start menu. Next time I wanted to launch the game, I saw the install button instead of play. At that time I had only several dozens of kB of free space on the drive where Steam and all its applications are installed, but even after freeing space on the disk (now I have about 80 GB free) the problem persists.But it's nothing unusual that I don't have space on this disk, since I use it to download torrents and other bit-by-bit downloads and occasionally I run to 0 B. And this is the first time this happened, so I don't think it's related.