I opened a saved game that I have been playing, then an error message popped up saying "Unexpected error occurred. If you're running the latest version of the game you can help us resolve the problem by posting the contents of the log file on the Factorio forums...". I have attached the error message, game save and log file below. I clicked "yes", and it took me to the location of my log and the game closed out.
The save was made loadable for the next version. (The paths just had to be rebuilt). I'm not 100% sure whether the cause of the problem was solved in the meantime, but I reactivated train/signal related consistency checks on saving for 6 future versions, so if it happens, we will find out while saving, which a) prevents invalid saves b) helps us to discover the problem.