Silversurfer
You need to check the logs. If they don't make sense to you, just throw them on some pastebin and paste the link here (10-20 lines will do).
It might be enough to just clear the GPUCache folder in: ~/.var/app/network.loki.Session/config/Session/GPUCache.
Try running flatpak run network.loki.Session --disable-gpu, maybe that will reveal more.