ᐅ Leaking internal roof gutter

Hello,
we have a leaking internal gutter where two terraced houses meet.

The roofer has been informed, but since the previous repair work appears to have been poorly done, I would like to get some advice from the community.
How would you seal the area shown in the photos?

What would be a reasonable price for a roofer to charge for this work?
The affected area can be reached without a ladder, and as you can see, there is no risk of falling.

I look forward to many tips.

Best regards from
Babenhausen

Without being able to see much in the photo. Safety regulations for craftsmen are quite strict, and for private individuals, it is often hard to understand why scaffolding or a safety harness is necessary. I would solder a metal sheet onto it.