I had the same problem.
Rendering animation from 3d keyframes worked fine, the same as you rendering into avi jpg or raw worked fine but ffmpeg just left a blank render window and nothing happening, no errors nothing.
As input I use a png image sequence of size 4800x6299 [px] (this is important).
I decided to add an effect transform with 2 keyframes at the beginning and the end with position x set to 0 (so no movement, just to get some keyframes to see what happenes) then I tried rendering and got an error saying: output dimension is not divisible by 2.
So I deleted the transform effect, changed height from 6299 to 6300 (1px is not going to make much difference anyway), hit render (this time again only the pure image sequence) and it started rendering.
So not sure if this can help you or not but I'd try checking that your output size both width and height are divisible by 2. If that doesn't work try adding some effect or something with keyframes and check if you get an error message as I did and try to check if yo uget any errors at the bottom.
This is after official 2.80 release so I'm nto sure if it's still relevant with your or not so I'm leaving this here if it helps someone else in the future.