Hi there
If you are using the business function fin_travel_1 then i would expect it to launch with FITV_POWL_APPROVER so that is quite strange! Depending on whether you use homepage framework or lpd_cust for the web dynpro, I'd be inclinded to check your entries against client 000 to ensure these are correct for FITV_POWL_APPROVER to see is this missing (check also does this exist in tcode POWL_QUERY).
The dump suggests a problem in BC-MUS-POW so in the powl itself but hard to say without seeing the callstack and where it dumps but it can indicate a problem with the POWL refresh or RFC issue (check if the url has the refresha=x parameter as mentioned in note 1542417 but this still would explain why you get the GUI screen!)
hope it helps in some way.
Sally