Jabber 9 and 10.5 error: Starting a feature set failed and Incorrect Username or Password

If you encounter this error in Jabber 10.5:

Starting a feature set failed

or this error in Jabber 9.3 or higher:

Incorrect Username or Password

when trying to sign in to Jabber using CUCM and CUPS (as opposed to Webex), and your typed password is actually correct, you may have a problem with your CUPS server and/or not have this user assigned to a presence server.

Test/verify:
To verify this is the problem,  try manual sign-in to CUCM (phone features only). If you can sign in to phone features, your problem is with CUPS. Make sure you have assigned this end user to a CUPS server and that the CUPS server is in a good state.