1. Change into avatar, 2. Kill all PC users with unity error.
I wish I had more info but I don't, it's happened to 3 people I know including myself, just change into an older (not recently updated) avatar, wait for it to initialize, and then everyone on PC just dies.
~~Only connection between the avatars is that at least 2/3 had GPU particle systems~~ (GPU particles are definitely irrelevant), re-uploading the avatar and making 0 changes to it (besides updating SDK ver and some other editor only packages, didn't think to test without changing that) fixes it, and makes it no longer a crasher.
(happens to all PC players who load the avatar, regardless of version)
Edit: just learned it also happens when avatar loads into menu.
edit 2: learning more, likely not shader related as loading into the avatar with fallback shaders enabled still causes crash.