My only other suggestion given you already checked for non-english and non-alphanumeric characters in relevant absolute paths would be to check folder and file read permissions and blocks. You probably already did this though lol, and setting up a vm should fix things anyway.
For completeness, your gpu needs to support 32-bit mesh indices (if it's modern it does, I only mention it since it's mod specific).
Sorry for the delay - I had to reinstall the VM a couple of times, managing to brick it during 3D initialization (Win11 and HDDs...).
Turns out it is "locale" related - namely regional formatting - if my format is being applied then I cannot use assets and models as they simply will fail to load (and I will have the exact same behaviour as before - some buttons not working at all).
If I switch to US formatting - mod fully works, models load and no issues are being observed. Except one - my values from previous save are completely out of whack - if I supplied (via slider) value like 6,8786845 (comma being the decimal point) - now it reads without the comma and goes "through the roof".
I am still looking to see what exact regional formatting option causes mod to stop operating normally (I tried reading the log file game creates but no luck so far) - so I have to do some trial and error until I find the specific option that causes the mod to break. Will post an update hopefully soon.
EDIT: ugh, it's a bit difficult to test - basically any "available" mangling with English US (or other English) regional formatting doesn't really cause the modded game to fail loading assets/models data.
But changing to any "comma as decimal point" regional formatting (so far I tried Albanian and Hungarian) - will "break" the mod from loading.
I see. The save slider value issue likely stems from the float.ToString() and float.Parse() functions now being conflated with different regional formatting if your prior save was on a different region.
Similar functions are used to process model data, which the modpackassets folder and modpackmodels folder uniquely contain.
I should be able to modify my code to be more portable across different regions, I erroneously assumed some of these functions by default did not utilize regional formatting.
Also, huge thanks for uncovering these errors. They would have remained unrecognized otherwise.
Hopefully I'll "quickly" check if some locale emulators can be used as workaround and update this post, but thanks for listening to non-US "complaints".
EDIT:
First - I checked some regional formats and some work and some don't. As I checked, Albanian, Hungarian and Russian regional formats break the mod, while Japanese, Spanish and Turkish regional formats will work with the mod. I haven't checked sub-types of those formats, only the "most expected" ones based on their name.
Second - Locale Emulator (from Github, for example, latest version 2.5.0.1) works as a workaround. I'll write down below how to use it "as portable as it can be":
1. Unpack downloaded LE (Locale Emulator) and launch LEGUI and close it.
1a. If LEGUI doesn't launch normal window - launch LEInstaller and it will prepare the environment - and immediately close it (alt-f4 is sufficient) - now relaunch LEGUI to make sure it loads as in step 1.
2. Drag modded game executable onto LEGUI exe and click "Create Shortcut" - if nothing else was modified - then LEGUI will create(update) specific config/shortcut for the game and immediately launch the game with parameters set, namely Japanese locale (that works with the mod).
3. Now the game can run with all the mod's features working (assets and models included).
Now for the testing if some regional formats will break the mod - in step 2 change the location to something that doesn't work, i.e. Hungarian (Hungary) - and game will behave exactly as I described earlier in this comment thread.
