Expense Report Error Out

A few times in a week, we have Expense Reports getting rounted to sysadmin's queue with the following message:

An unspecified workflow error occurred while processing this expense report in Oracle Approval Management. Please refer to the Error Message item attribute in the workflow monitor to determine the nature of the error.

All we need to do is to push the "Find Approver" button, and it will go to the approver. Does anyone know what causes the hiccups?

Thanks,

Ling