Anywhere in the "outside" the background only renders squares/blocks of colour instead of the beautiful backdrop.

Sky islands is especially ugly, though I forgot to screenshot those areas since I was busy installing/reinstalling and messing with resolutions to no avail.

In the previous 1.9.04 there are no such issues, see screenshot.

If my screenshot doesn't attach properly, please see the Imgur links.

https://i.imgur.com/7iGRM0G.png
https://i.imgur.com/hE3Tmcs.png
https://i.imgur.com/XQ8H172.png

Even though the latest game files on GOG are named v1.9.15.3, in the options menu in the top left the version is v1.9.15.b which is probably adding to the confusion!

I'm having the same issue. Did you find any solution to this?

I haven't scoured the game yet for the graphical issue, but when I installed it again I did notice that for both the base game and DLC the menu still has the wrong version listed of v1.9.15.b. I've contacted support about this and it's worth it if you do the same, so that they'll know it's not an isolated issue.