I am simply trying to get the sample PopUp app to work..
I get the error attached, which appears to occur when Register Terminal is called

What could be causing this?

I have tested the login details and IP address used using the Dashboard and with that I am able to
START APPL SESSION
GET DEV ID
START MONITORS
REGISTER TERMINAL (works with Media Control:No Media and in all dependency modes)

The response indicates the RegisterTerminal message has unexpected fields in it. The only guess I have is one of the data values has "<" or ">" in it confusing the parser in AE Services. I would carefully inspect the data values that you have put in the configuration file looking for a mistake.

If that does not give a hint, enable XML tracing in AE Services (there is a FAQ describing how to do this), and look for where the message is received by AE Services. You will find the same error message you post below in the dmcc-trace.log.0 file. Inspect the logs around receipt of the RegisterTerminal request and see if AE Services provides a better indication of what portion of the request it thinks is flawed.