otter does not watch videos on youtube
This is older but basically says that some videos need gst-libav and gst-plugins-good to play with Otter and Qupzilla. I checked my Sparkylinux and have the gst-plugins-good installed by default but the gst-libav plugin is not installed. Maybe you need this in your system for Otter to work correctly.
https://forum.manjaro.org/t/solved-qupzilla-and-otter-browser-wont-play-video/2525/37 gst-libav plugin -
https://gstreamer.freedesktop.org/modules/gst-libav.html