Chatted with the core and messaging folks about this particular error - they think you may need to contact TAC on this one.
One thing they did suggest was to install CiscoUnity3.1(5)_ES52. CSCed89501 (which was fixed in that ES) adjusts for a similar exception in the ExchangeMonitor. They don't really think you're hitting CSCed89501, though, because it was limited to earlier ESs in the 3.1.5 branch. However, the work for CSCed89501 improved exception handling in the ExchangeMonitor overall which would certainly help run this down.
You'll want to contact TAC to get hold of that ES and go from there.