Beta 6 of X-Plane 12 Early Access is available on Steam & through the Laminar Research installer. You should be prompted to auto update the next time you launch the sim, if you haven’t already. Here is a link to Early Access release notes.
This version should fix the problems launching via Steam on Linux. It also has more fixes for magenta NaNs, so hopefully there is more or widespread relief on that front.
We’ve heard multiple reports about external visual networking being broken since beta 4, and we are currently investigating.
Finally, X-Plane 12.00 beta 6 includes internal changes to support the .ktx2 texture format. We are working on tools and documentation to go with these engine changes, and will post detailed information on how to use .ktx2 once the specs and tools are complete.