Alright lads, I got it.
Just fixed my savegame with a tool called SEToolbox

1. Go to this website:
https://github.com/mmusu3/SEToolbox/releases

2. Download the latest zip

3. Unpack whereever you want

4. Start the SEToolbox.exe

5. The tool will say that SE has a new version and that you need to accept. Do so.

6. Start the toolbox again.

7. Click on "File", then "Open" on the upper left, open your savegame. MAKE A BACKUP IN CASE YOU MESS SOMETHING UP

8. Do NOT click "repair", click "open" instead, right side.

9. Now you gotta play detective. You have a list with all the grids and objects in your world. Find the grid(s) with the blocks which are breaking your save. So far we know that Steam generating reactors are affected and some people reported also that decoys mess things up.

10. When you selected the grid, click on "cubes" on the right window. Look for the breaking cubes. Click on them, delete them via the red cross on the left side.

11. Save the edits - again, use this only when you made a backup of your savegame before! The edits you are doing here are surgically effecting your savegame and you risk destroying hours of work with that!

12. Run SE, load your save and see if there is still something breaking your savegame. In my case, getting rid of the reactor was enough. I can safely replace it now.