error.log:
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Timestamp   : 9.1.2012 15:15:01
Error level : ERROR
Message     : Lua hook OnAction caught ELuaStateException!
Call path     : beings[].OnAction
Call params   : ()
Error message : Lua error : lua\ai.lua:709: Object expected as parameter 1!
----------------------------------------------------------------------
----------------------------------------------------------------------
Timestamp   : 9.1.2012 15:15:01
Error level : FATAL EXCEPTION
Message     : Fatal exception encountered
An unhandled exception occurred at $00464249 :
ELuaException : LuaError: beings[].OnAction -- Lua error : lua\ai.lua:709: Object expected as parameter 1!
  $00464249  TDOOMLUA__ONERROR,  line 589 of src/doomlua.pas
  $0044F8B7  TLUASYSTEM__PROTECTEDRUNHOOK,  line 476 of e:/Projects/fpcvalkyrie/src/vluasystem.pas
  $0046024C  TTHING__CALLHOOK,  line 152 of src/dfthing.pas
  $004A6D10  TBEING__ACTION,  line 1058 of src/dfbeing.pas
  $004A6E2C  TBEING__CALL,  line 1076 of src/dfbeing.pas
  $0041F8EE  TDOOM__RUN,  line 457 of src/doombase.pas
  $00401FFC  main,  line 61 of src/doomrl.pas
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Screenshot and save file (hopefully) attached.
I was playing HntR Angel of Shotgunnery, nuked Mastermind with overloaded nuclear BFG and proceeded to attack JC with my elephant gun. Just as he died, this happened. I can't recall anything out of ordinary (except the fact that I now routinely reach JC, gotta start with HMP soon =)
Edit: Damn, it seems the save file didn't get uploaded corrcetly and is gone for good - I played it again and got my ass handed to me by a morally wounded JC.
This time around, however, JC started spawning tougher enemies, in the first attempt he only ever spawned lost souls (which explains why I managed to take him down), so maybe that's related to the crash. In the first game he spawned lost souls 6+ times, while now he pretty soon switched them for cacodemons, knights, barons and then revenants.