The most common cause for this has been fixed on the impostor generator side. It actually caused the impostor to be disconnected from the animator entirely, so all parts were effectively immobile. It'll be a while before all affected avatars are automatically regenerated, but this issue will get less common over time.

In testing, we found a couple avatars that still had arms in a t-pose after regenerating, but other body parts would animate. In both cases, the skeletons were rigged in unusual ways which caused the impostorizer to treat the arms as part of the chest. We're now tracking that issue separately.

I keep running into this problem as well. Thought it was my avatar at first, but after testing on android and PC, it's definitely the impostor that's stuck in T pose.

I've only had android people tell me that my impostor is stuck in a T pose (tho don't know if impostors are a thing on PC as well). I do use FBT, but when testing myself with 2 accounts (one on standalone and one on PC) it seems like the problem also happens when you're just a PC player (no vr no FBT). Only fix is to tell the other person to disable my avatar completely or turn it on, the shield setting with the impostors does not work no matter what avatar I use (public or private)

I've also seen it happen. I can't give any IDs, but going around public worlds and setting everyone to show an impostor, it's bound to show up.

Looks like not only FBT users,

happens for non-vr desktop users too with sdk2 avatars from 2020

I would like to upload more, but...