I have CCM 4.0 installed and Cisco IP communicator. After starting the communicator, Cisco IP communicator displays error message: "Registration Rejected: error mismatch".
firts thing I would suggest is to delete the previous entry of ip communicator and re add it
if this does not fixes the problem try the following
Go to the route plan report of the publisher, and search for all of the unassigned DNs. If there are any, delete those extensions.
Verify if Auto-registration Disabled on this Cisco CallManager is checked under System > CallManager > Server in the CCMAdmin.
Choose the respective Cisco CallManager group, normally the default, and verify that Auto-registration Cisco CallManager Group is checked.
Finally, choose C:\Program Files\Cisco\TFTPPath and look for the xmldefault.cnf file. Open the file with Notepad, and make sure that the Cisco CallManager group is there. It appears as the IP address of the servers group at the beginning of the file.
Check to see if a Certificate Trust List (CTL) is used.
Check for any replication issues.