Just wonder if you have tried to find a corresponding WFERROR workflow via WF Status Monitor Web interface.
I think there should be some information under the sections there all attributes for the WF are reflected.
Let us know if you tried it already and we will come with something else,
I returned 'ERROR' from the function attached to the even subscription and set the error message in p_event.setErrorMsg().
I also inserted SQLERRM into a temporary table in the exception block of my even subscription function.
These two things helped me in figuring out pl/sql errors occuring during event subscription.