Your config file appears to have become corrupted. Try deleting the ".minecraft/config/replaymod.json" file.
This worked, thank you so much! To finish it up I have one last question. Do you happen to know a good tutorial about ffmpeg and installing it because that's where things went wrong last time if I'm right. (Sorry if this is not the place to ask).
1. Download ffmpeg (should come as an archive of some form, e.g. .tar, .zip, .7z or similar)
2. Unpack downloaded archive
3. Move the resulting folder (which should contain a "bin" folder) to ".minecraft/ffmpeg" such that ".minecraft/ffmpeg/bin/ffmpeg.exe" exists
4. Done