The URL that you would put into your browser as well as what you put as the URL in the Softphone config would look something like this:
http://server/CiscoIPSoftPhoneInstall/getIP.asp
Note that this is not case-sensitive. If you go to that URL and still get the 500 error, and you have verified that the getip.asp file actually exists in the correct location on the web server, try going to that server and restarting the IIS Admin service and do it again.