I had a similar issue happen, when I went EVA on a moon mission, and when I boarded it left a GHOST KERBAL. Soon after I had the described glitch where I could not buy any kerbals and it showed a false limit. I went through the save file and found an "assigned kerbal" with an idx of -1 but that didnt fix the issue. What did work, was just going in and deleting ANY of the kerbals in my roster. That returned things to functioning like normal... so I have no idea. Hope this helps someone else in a similar situation.
