The reasons for this error are manysome!

Is it related to the WebLogicServer targets? Try this

Is it related to a listener?
Under the Errors sub-tab, the error message reads “Target is in broken state. Reason - Compute dynamic property takes too long”?
From:
Oracle® Enterprise Manager
System Monitoring Plug-in Troubleshooting Guide
Release 12 (12.0)
E10314-11
November 2010

Cause
This behavior occurs when target, agent, or both and/or target system become slow or heavily loaded and computation of dynamic properties take longer than the default timeout value. Workaround
Execute the following command:

 emctl reload agent dynamicproperties <target name>><target type>

OR
You can alternatively resubmit the target instance properties:

• Click on the Monitoring Configuration link (under the Related Links section on the plug-in target home page).
• Re-enter the encrypted properties.
• Click OK.Try re-confirming the listener configuration page (without making any changes):

OR
<code> Targets : All Targets Search for “Listener” Click the radio button next to the listener that's not up and click the “Configure” button. Check the details are ok and click “OK”