Had it happen a few times now, no idea what's triggering it. Relevant line from output_log.txt When opening Astronaut Complex: When focusing on a vessel holding the kerbal with the bad index in the map view: It happens with both modded or unmodded install. Can provide save on request, but I only have a modded install at the moment. How to manually fix it You need to find which vessel holds the bad Kerbal, you can do this by selecting each one in the map view until things break. Once you know which vessel is bad, exit KSP and open the save file. Find the relevant vessel section in the savefile: Find all parts of that vessel that contain crew , write down each crew member (Your crew will be different from the example!)Search for each crew members roster in turn -1 is not a valid idx for an assigned crewmember, it needs to be set to a valid range.For example if you have a cockpit that can hold 3 Kerbals, make sure the crew members seated in it have their idx set to 0, 1, and 2. If you have a single crew pod, the one Kerbal in it should have his idx set to 0. If you have multiple crewed parts, each part has its own idx, cockpitA will have Kerbals with idx 0, 1, and 2. And cockpitB will have Kerbals with idx 0, and 1 (For example) Remember, idx start at 0, not 1! Repeat until you've gone through all broken vessels, and all crewed parts on each vessel. Start KSP, load save, should work.