HELP! Organizer, please delete this thread, issue has been solved / added to article.
I saw this: (didn't really play myself yet)
https://steamcommunity.com/app/337000/discussions/1/355043117528654702/#c352792037311648900
It says that DX:MD seems to completely ignore one's Steam cloud sync preferences. Even if they're turned off (globally and also in the app settings), after about 148 saves the cloud space says about 950MB are used and about 9MB free. And that's when saving stopped working altogether. (Btw. a savegame is about 50MB in size.)
The poster tried to edit remotecache.vdf to refer only to the most recent saves (after he removed the rest of them) in the steam\userdata\(u_id)\337000 folder, but this didn't help. Tried clearing all caches which successfully emptied the cloud storage, and the game then said that there were indeed no saves at all, even though the few most recent local saves were there. (Again, both cloud sync settings were set to OFF.)
The guy then started a new game and created a savegame or two. The cloud info showed that apparently, those files were then being uploaded into the cloud even though that should be IMPOSSIBLE. Starting Steam in offline mode didn't help, because now the newly created savegames could not be loaded - that seems to be bound to cloud storage somehow.
The guy can't continue playing the current campaign because he can't use those save games because the cloud is full. "Editing" the cloud content does not help - only wiping works, but then obviously the campaign is gone.