The main menu with the Replay mod installed (Image via Mojang) Now that you've got both Replay and FFmpeg installed, you'll need to launch the game using the CurseForge or Modrinth installation ...

It also contains the output of ffmpeg saying: Unknown input format: 'rawvideo' It seems your ffmpeg install is not compatible. So replay mod does not know what to do an crashes.

I do not know how it could be fixed other than flatpak shipping an updated version of ffmpeg or Replay Mod asking flatpak users to run sudo flatpak override LAUNCHER_ID ...