I am experiencing the same problem (Singleplayer; PC; no crashes). I have finished three maps and working on the fourth, Seashore. I replayed the Island map and came back to Seashore and got this error message:
Unable to load save data from level SEASHORE
I saved the file if you need it.
Numerous other people are having this problem on the Steam forums. A save game can represents hours of effort. People will be upset and stop playing the game. Is FOCUS Support watching these forums?
I do not want to lose hours of work. So, I have started backing-up my save games. If you would like to do the same, here is the location of the save folder:
They are saved using the old map names. So, Island is volcano.sts, Seashore is coast.sts, etc.