DeerStalker

Frontier does not have oxygen in Version 1.6.3 via CKAN. Monkeypatching Frontier.cfg line 164 to oxygen = True and deleting the cache works though

I encountered this bug too by going out of physics range via an EVA on Minmus and then coming back. Workaround to save your game again: destroy the Vessel. Probably just the buggy command pod would be enough. I did a rendezvous with my spacestation in LKO before the end of a mission, EVA the kerbals and science to the station and then let the mission-craft disintegrate in the atmosphere. As soon as the buggy pod was destroyed I was able to save the game again.