IIS 5 & IIS 6
Error logs in Windows Server 2003 IIS 6.0
Re: Error logs in Windows Server 2003 IIS 6.0
Mar 31, 2009 04:43 AM|Paul Lynch|LINK
Just a thought - its possible that their attempt to connect to your server is being met with a 500 error which originates from some other network device sitting between their client machine and your server.
I would ask them to capture a network trace using Wireshark which should reveal the true source of the error.