Hi Fabian,
In softphone mode, the Jabber SDK requires a specific device configured in CUCM to successfully SIP register. The device must be of type CSF (Cisco Unified Services Clients Framework). Default device name is ECP{user} by convention, where {user} is the CUCM end user id passed to the registerPhone function. A different device can be selected by the application by specifying a "devicesAvailable" callback to registerPhone (see the cwic API documentation).
In order to register in softphone mode, the Jabber SDK first downloads the device configuration over TFTP (port 69) or HTTP (port 6970). Are these protocols/ports allowed between your client and CUCM ?
You should find more details about the exact error in your local softphone.log under C:\Users\{wuser}\AppData\Local, where {wuser} is the Windows user is.
Hope this helps, Thomas