Reload the sdk. If that doesn't work, make a new project. Also, I suggest downloading VCC to avoid this issue in the future if you haven't already. What this is, is a script causing issues. Make sure when you get VCC that all you're importing is Poi (or whatever shader is being used), vrcfury (if the avatar requires it), and then your avatar. Make sure to import one at a time rather than all at once as that usually confuses Unity and causes it to break. Make sure to also check the avatar file and untick poi or the sdk if it's included in the package as that will cause it to break.
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