FWIW, part of the reason we put Apps with Workers on hold for a while is because situations like these are really difficult and frustrating to deal with at the moment. Neither you nor the app developer have a lot of visibility into why the app is failing. This is really our fault, not yours nor the app developer’s. This isn’t an experience we’re happy with, but we’re not quite ready yet to dedicate the time needed to make apps work well. (We’re getting closer, though.)
I believe the app you are using also allows you the alternative of integrating with them directly in your own worker. That might be a better option for now, as that way you’d be able to see the exception being thrown and fix the code directly.