4.8.4.0 broke playback and had to uninstall it and reinstall 4.8.3.0
As soon as 4.8.3.0 was reinstalled, everything worked perfectly fine.
I was getting an error when trying to play anything that said "Playback Error No compatible streams are currently available."
uninstalling/reinstalling was super simple for me. just make sure the backup and restore plugin is configured just in case.
uninstall 4.8.4 and install 4.8.3 -- here it is for DSM 7.2+ https://github.com/MediaBrowser/Emby.Releases/releases/download/4.8.3.0/emby-server-synology72_4.8.3.0_x86_64.spk
if things don't fire up as usual after installing 4.8.3, your backup should sort it out but I didn't even need to do that