I had same issue (STEAM version), here's how I fixed it.
1)
Go to
SteamLibrary\steamapps\common\ARMA Cold War Assault\Users\<YOUR USERNAME>
2)
make a copy of UserInfo.cfg
3)
Open UserInfo.cfg in Notepad.
4)
Find:
terrainGrid=<some number>
5)
Change <some number> to 50.00
(I also changed frameRate to = 144)
6)
Save
I was then able to play the mission.