No the AHP PC's internal ip does not work either from another PC on my home network.
I check my router assigned DHCP often and it has not changed for two+ months. Same with my external ip. Remember I had everything working excellent on WiFi and 3G with port forwarding using the old iwatchmobile V3.285 until this Pro upgrade.
I think you have to be getting close to solving the problem. You have iWatch PRO working on your AHP system, so everything is in order there. You have your iPhone receiving a 500 error message from the system, so it seems like you're making a connection. I don't know why you're not getting a more descriptive error. My first guess at this point is that you have 'Use Wi-Fi Security' checked in your Invite Cell Phone dialog and you're using a URL without the security key. If so, try unchecking that box and refreshing your iPhone's browser.
Also, try '
http://my external ip:8777/iwatch' instead of '
http://my external ip:8777/iwatchpro'.
The next thing I would check is making sure you can access the Invite Cell Phone dialog for both iWatchMobile and iWatchMobile PRO. This is just to check that both are registered.
And last, and this is a long shot, check that the file version for c:\program files\common files\x10\common\x10net.dll. It should be release 294. Check that the c:\program files\common files\x10\common\WebApps folder is present and contains xphone.html and xphonepro.html.