ᐅ Flat roof with parapet wall on garage leaking

I have a flat roof with a parapet on top of my garage. A metal box runs through the parapet, directing water into the gutter. The problem is that the roofing felt is no longer watertight at the transition to this metal box. It seems that the roofing felt was not extended overlapping into the metal box but ends exactly at the edge of the metal box. The roofing felt there can be lifted (see photo with plastic scraper), and water appears to be finding its way inside underneath. Now, whenever it rains, water leaks into the garage. Sealing it with bitumen does not work; I have already tried that. Does anyone know how to permanently seal this junction between the roofing felt and the metal box?

Thanks for the advice!