The reasons for this error are manysome!

Is it related to the WebLogicServer targets? Try [[WebLogic Server reports "target has encountered metric evaluation errors" in Grid|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:
<code>
 emctl reload agent dynamicproperties <target name>><target type>
</code>
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"