weird new crash with reinterpret_cast<>

I was trying to make my code build-compatible between 32- and 64-bit, so I
put the following in an ISAPI filter 6-9 months ago:
HTTP_FILTER_CONTEXT *pfc; // parameter passed in
try
{
....
    __int3264 ctxPtr = reinterpret_cast<__int3264>(pfc);
....
}
catch(_com_error &e)
{
    std::stringstream message;
    message << "COM exception in OnPreprocHeaders(): " << e.Description();
    WriteToEventLog(message.str(), EVENTLOG_ERROR_TYPE);
....
}

The _com_error handling code had to do with other code around.

This was working just fine for 9 months and just today it started crashing
on the reinterpret_cast<__int3264>(pfc) call. Somewhere in
reinterpret_cast<__int3264> an exception is handling, but when I get into my
catch block, the _com_error reference isn't a _com_error reference but an
HRESULT, so the catch block blows up trying to look at e.Description().

Any idea why this cast would suddenly blow up? The value of pfc looks to be
a pretty vanilla value, nowhere close to a sign-extension boundary or
anything else...

Thanks
Mark