Hello,
Unfortunately this is a known problem, and we are aware of it. Currently there is being looked into a more solid solution, until then, please use the workaround given below so you can access your save games again. My sincere apologies for the inconvenience this issue has caused you!
1. Open Steam and go to your Games Library
2. Right click on Ancient Space and select "Properties"
3. Navigate to the "DLC" tab
4. Uncheck "Bonus Content DLC"
5. Go to the "Local Files Tab"
6. Click on "Verify Integrity of Game Cache"
Then restart your game and you should be able to access your previous saves again!
Note that any saves started BEFORE you unchecked the Bonus Content DLC will not be visible without the Bonus Content DLC Enabled!
