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Please name exact versions.
sfall is ddraw.dll in game directory.

What's your RP, FO2tweaks and sfall versions?

If you remove fo2tweaks, do saves work again?

Both UPU and RPU are updated to version 21. The only change is sfall version update to 4.3.0.1, which fixes some issues discovered in 4.3. If you are playing UPU or RPU v20, you can continue, but make sure to update sfall.

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Well, unfortunately I can't find the cause. Probably some conflict on engine level. For now, the best I can suggest is to keep FoW disabled.
Maybe later, if hi-res patch get re-implemented in sfall, this will be solved.

Yes. I'll look into it.

I can't confirm. Using your inis:

OK, can you attach full fo2tweaks.ini, ddraw.ini and sfall-mods.ini?

There's a bug in LOS checking.
Extract this into data/scripts, should work.

attach a save and a screenshot