I'm having a rare failure in my (multi-threaded) web server.  The fault could well be mine, but as the failure occurs in rexx.dll I'm having trouble getting a handle on it.  Any pointers as to what might be causing it would be really helpful.   The details:
  Exception: 0xC0000005
  Location: offset 0x00039966 in rexx.dll
  Rexx version: REXX-ooRexx_4.1.0(MT) 6.03 5 Dec 2010
Running on Windows XP (fully up-to-date).