Hmm, unfortunately I'm another long-time player who has recently started suffering from the same plague. Symptoms are exactly the same, which made me come to the conclusion that the crash on saving is OBSE-related (also the game creates an .ess file but not an .obse file in the Saves foldes). But to make things complicated, I quit using P1KeyChain years ago, so it's not exactly the culprit - or not the only one. I have tried to solve this mystery on my own, but honestly I'm out of ideas. I have played and carefully modded Oblivion since its beginning, and as a well experienced modder I seriously doubt it's some noob error, although I wouldn't exclude that entirely. I wonder if anyone has ever come upon a good idea what might be causing this problem that came out of the blue, without installing new mods, changing settings, etc. A funny thing is one day I could go back a few saves, and load one of them, save, and be good to save again. Next day I load the same save as before, and I can't save from it any longer. Huh.