cannot save games.
 

I have seen similar posts. It is late 1943. When I start a mission, I can save it and resume it. But once I have spent some time at sea, I can save it, but when I try to resume it, I sm unable to do so. If this a game issue? Am I stuck with have to restart the game in such cases?

You mean corrupted saves? Yeah that's a recurring problem for some, known bug.

If you can't load saves from a patrol, you should still be able to load the auto-save when entering and leaving port. At least that way all you lose is half a patrol, instead of your entire carreer.