...here's the fix, straight from the chat server guys. It worked for me.
Thank you for contacting ParaChat Support. You will see "Site ID does not exist on server" when you modify your ParaChat iPad App access information, but do not close the app and re-launch it. When you modify the access information, close the app, and re-launch it to log in. iPad supports multitasking, so you need to close the ParaChat app from the multitask dock. To do so, press the Home button twice to view running apps, and scroll to the ParaChat app. Tap and hold the app icon until the minus sign displays, tap the minus sign to close the app, then press the Home button to close the multitask dock. Re-launch the ParaChat app to log in.
Personally, I prefer the browser-based mobile device user interface, which is also designed for iOS devices. Just visit this link to log in:
Thank you for choosing ParaChat as your chat solution, and please let us know if you have other questions.