No problem. I think I know what happened now.

I usually play offline, without steam verification, checking for updates only now and then.
My playername changed from <steamnick> to <unknown> (don't know why).
So the program is probably missing the save because it looks in the wrong directory?

Exclusively? Sounds wrong, at least if you have SP content.

Steam offers local play, so usually there's also local saves. I used to play on the train during commute (this game would do very well here),

No trouble, this is Beta. Anyway I must have been tired yesterday, ofc there's a local save, I used it.
I found the issue.

Savegames are in a Document folder (My Games\Offworld), which is unusual.
But my folder is usually encrypted and must be mounted separately. Windows can use several physical locations for this logical folder.
The encrypted version has priority, if it is off then Windows uses the default folder.

When I quit I shut down the computer instead of just going to sleeep mode; next day I didn't mount documents first. Explains the missing save game.
I also didn't start Steam, just used a link on the exe. Maybe this caused the nick to be changed to <unknown>.