When an update is made you may get a desync message when trying to play co-op or a save game may give you a warning about a previous version, and your save game fails.

Desync Co-op:
To fix this issue go to your mowas2 dir folder...
C:\Program Files (x86)\Steam\steamapps\common\Men of War Assault Squad 2\mods

Find the folder named "The Beach". And Delete it.

Then, come back to "The Beach" workshop page and "Unsubscribe" and then "Subscribe" again.

Doing this forces an update of the downloaded workshop content tucked away in...
C:\Program Files (x86)\Steam\steamapps\workshop\content

When you open the game again (if you had "The Beach" mod activaterd), you will get a message effectively saying it cannot be loaded. Simply go to options mods and reactivate the mod which will copy the newly downloaded content into...
C:\Program Files (x86)\Steam\steamapps\common\Men of War Assault Squad 2\mods

All parties playing co-op should follow these steps to ensure everyone has the same version.