Mounting Old Agents fails with "EM has too many disconnected agents manually mounted, please un-mount the unwanted agents".
Try increasing the value of "threshold value" in the "apm-events-thresholds-config.xml" on the EM and recycle EM.
Limits the # of dead/disconnected agents that are mounted
state to minimize
the runtime memory of EM.
For more information, please refer CA APM Configuration and Administration Guide under " Introscope Clamp Properties".