I've updated the game using a new patch. After starting it I loaded my save from version 1.3.1.5. For some reason my controls got reset to defaults (probably irrelevant), and after I went to the save file folder to check if anything changed I saw that new files appeared: "Crashes" folder (empty), a "user1_1.4.2.4.dat" save file (around 30KB bigger than user1_1.3.1.5.dat), and a log file telling about unloading 47 assets due to not being used, and some other stuff.

I've also downloaded GOG Galaxy, and verified the game files just in case, but this gave no results as well.

Did you experience the same issue? Does anybody have an idea how to play the version 1.4.2.4 using previous version saves and without replaying everything?

Okay, I figured it out more or less. The content IS in game and it is available to me BUT, the crash logs still keep appearing for some reason whenever I start the game. All I did was pick up the 4th simple key, so I can't tell if everything is working the way it should or not yet.