Internal Salesforce Error: Workflow Email Alert

Hi All,

I have been trying to find the cause of this issue since long and still dont have a clue about it. :(

I have a custom object on which some trigger is written. The code sends an email to user when a new record is created, using workflow. This works smoothly but when APEX test case is executed, it shows an error message and test cases fails. The error looks like : Internal Salesforce Error: 791389547-89 (-1559824086) (-1559824086)

Deactivating the worklflow makes this works fine. I have tried all the combinations of workflow values and a blank email template as well.

Is there any limitation in Salesforce on workflows and emails. Please suggest.