Render Output is Just Black

Hy there folks.
Still working on my Intro and got it running so far, but now i have a Problem with the Output.
While during the Rendering all seems well and good, there is a Problem with the Output Image… is Plain Black.

Since iam New to this, i struggle to find the Problem and hope somebody here can tell me what went Wrong.

I made 2 Szenes in Cycles, and 1 in Blender (pure Smoke)
I used a Mixer in the Node because i found a Topic where somebody had a Blenderfile where this was the solution.

Since i Still cant Upload files, i hope the Link works

I don’t have experience with compositing but it seems to be a problem with the add node and “dissolve” scene.Removing add and the dissolve scene and plugging it into the compositor from there gives me this.

Try using ‘Alpha Over’ instead of the ‘Add’ node at the end.