To add, myself and a friend are currently experiancing a very similar issue. While trying to run a CMBS quick battle on the Steam version of the game we are able to start a quick match, buy our forces as normal and proceed to the first turn of the game. However the first player to go (we have both attempted this separately) can input their orders as normal but when saving the game it will create a file around 1kb in size for the save file. Any attempts to open the file result in an immediate crash, both of us have attempted to create the mission and each player had the same issue on both our machines.
I have attempted by myself a number of times to recreate the game using an altered version of the map and game settings to no success, for whatever reason the first turn will always result in a corrupted file.
We were attempting to play on a slightly modified version of this https://www.thefewgoodmen.com/tsd3/cm-black-sea/cm-black-sea-add-ons-maps/chernova-hirka-mastermap-v1-0/ however i did also make a game based upon an unmodified version of the map just to make sure we had not made any issues while tweaking the map which also resulted in the same error. I had no issues creating a standard PBEM game of one of the games pre made battles but this is the first QB game we had started on the Steam version of the client. We have played similar sized scenarios before in CMBFN and both our machines are rather powerful so i had not assumed this map would have been an issue based on that.