This can be one of the headache error that you could experience as your ESM could be hitting with a constant flood of the following error:
[2015-09-03 10:44:14,555][WARN ][default.com.arcsight.common.introspection.dependentvariable.DependentVariableIntrospector] DV value cannot be set for a security event
Start off by disabling all the Data Monitors for all of your custom DMs or most recent ones. Remember to do a tail on the log as you want to monitor which DM is causing the issues. If you can't determine when disabling them then it could be more than one hence, start up one by one and see which one trigger that error.
Next, follow the same steps for rules if DM is not the culprit.