Were you using any other mods?
How long did it take to "render"?
How long was the camera path you were rendering?
Did you install ffmpeg correctly?
Which version of forge and replaymod do you have?
Finally, what OS do you have? (windows, mac or other?)
Nope, I was only using RM and Optifine.
I don't know, because the screen stops wile rendering.
It wasn't so long, only a few seconds.
I have my questions about if I install it correctly, but after watching 10 different tutorials, I think so.
11.14.1.1337. (I think is too old)
Windows 7.