Hello, I got help on discord on how to get around this issue. Apparently, one of the PR broke the save feature ( https://github.com/FAForever/fa/pull/6647/files ).

To fix this, you need to find "/gamedata/lua.nx2". Create backup for safety and rename the file to "lua.zip", find and extract file "userInit.lua". Inside, find this line (should be near the top) "doscript '/lua/ui/globals/InternalSaveGame.lua'". Delete it or comment out, save the file and put back into lua.zip. Then rename it back to "lua.nx2". After doing this, saving and loading started to work again for me.

I have uploaded lua.zip file after the changes I made, you can use that. Just rename it to "lua.nx2" and paste into "/gamedata", just be sure to make backup.

lua.zip

When I launch the game from "C:\ProgramData\FAForever\bin" and try to save skirmish game, I have error "Save Failed". Launching the game from steam (no FAF) allows me to save and load normally.

I do not have Controlled folder access and added folders related to Supreme Commander to Windows Defender exclusions. Still, I cannot save the game.

What I want to do is play campaign and be able to save. I followed this guide https://forum.faforever.com/topic/4702/how-to-play-the-supreme-commander-campaign-offline-and-coop-maps
I can start the missions with no problem, just that no matter what I do the message says "Save Failed" when I try to do so. I have no other ideas how to solve that, I have no external antivirus, just Windows Defender. I do not have this problem with other games.

Edit: Found solution, check my reply below for specifics.