My mistake. I checked the PHP error log and it didn't show any errors within the last 2 months.
Yep - there's no chance that error won't be logged. If you can't see it in the log, you're looking in the wrong log.
I have checked the php error log, the http error log, the iis error log. Nothing has shown any evidence besides the error entry posted above. Where else can I look to find some evidence of this problem?
It's always possible error logging is switched off somewhere - I've never used IIS so I don't know - however, there is plenty of info in Google land about 500 errors (and 500 errors and PHP). E.g. http://support.microsoft.com/kb/943891