I have an app user in the dev environment that I can use just fine to use the Presence Web Service in both SOAP and rest. The same application user (same roles), always gets me an error 400 in the productive environment.
The developer guide never says which configuration is necessary for the application user to even log in.. so I'm wondering what I'm missing here to get this working.
There's no special configuration needed. The documentation should tell you all you need to know to log in an application user. https://developer.cisco.com/site/im-and-presence/
If you're getting an error with the REST request, I recommend trying to do the same operation in Soap to see what happens. I use SoapUI extensively for this kind of thing. Create a project with the WSDL and play around with the various operations. If you're still getting an error with SoapUI, post here again with the response and let us know.
Just in case, here are the two groups I have configured for my application user:
Admin-3rd Party API
Application Client Users
That's it. Admin-3rdParty APi. Since it has no roles, I figured it wouldn't do anything.