There is a version dependency between webservices and ump. The webservice_rest package version needs to match the ump wasp version otherwise you will get the message that you have reported.
Please double-check the documentation for multiple UMP servers configuration:
If the issue still occurs please do the following:
On the UMP robot please open the controller GUI > Status > Installed Packages and check if webservices_rest is installed. If so, please make sure it's v8.51, as the wasp. If it's not there, please distribute it from the Archive.
Restart the wasp and check if the issue reoccurs.
If this doesn't work:
1. Deactivate the wasp probes on both UMP robots
2. Delete the <installPath>\Nimsoft\service\wasp\work directory and its contents on both UMP robots (when the wasp probe is restarted it will recreate this directory and repopulate it with current information)
3. Activate the wasp probe on the primary UMP robot and when it has a PID and port, activate the wasp probe on the secondary UMP robot
4. Clear the browser cache from the beginning of time then try logging into the secondary UMP