Path Diagnostic is running only in 1 direction on Dual Ended Path.
Error generated - Diagnosis failed. Required port is unavailable
Ensure the custom ports configured on this path used for UDP source and destination are not the same, This information is visible from the far right pane on the path:
In-turn, return diagnostic will try to establish a connection using the same parameters, but as it is already using the port of 25000 for the continuous path monitoring, it cannot be used for to spin up for the return diagnostic.
Changing the ports so they do not match will resolve this issue.