If it helps, I managed to load a corrupt save and get back to base, working entirely from map view, so my sub didn't have to load.
This is what has happened to my sub:
This is a new career, however, so it wasn't a fluke last time.
It appears to have become an issue after some level of tonnage sunk (around 400,000 to 600,000), which I've heard whispers about before...