Hi,
We agree that the output triggered and its in SOST. But still the message is showing as red. Select the output type line item and click the "processing log" button as shown below - this will show why the message is showing as red.
I think it could be due to a bug in your custom driver program. You may also check with the standard print program SAPLFM06P instead of your custom program and test the behavior!
Regards,
AKPT