Very temporary, at best regarding the cement powder. And that is if you found the pin-hole leak or crack.
Usually when a flat roof intersects a sloped roof, especially when they are 2 seperate building structure, expansion and contraction will put stress on the materials making the transition and not too many guys use the proper combination base flashing materials for this junction, but instead, use the same plies of hot asphalt and felt papers.
Check along the transition and gently feel for any hollow spots ant the see if any horizontal crack have begun to form, without splitting completely through so far, but wide enouge of a fissure to allow downward flowing water to enter in.
If that is the case, I will get back to you with the most appropriate and inexpensive repair possible.
I am suspecting that will be the problem, so I am steering you to 2 other threads that provide for a repair with roofing cement and a reinforcement membrane. Ed
Roll roofing again LOL!
This 2nd link has a much more detailed explanation of how to go about doing this type of repair, but read through them both, since their is some handy tips in each one. Ed
Roll Roofing Repair