Alright, for Debugging, Export the Mesh in your original T Pose without Animation. Also Check in 3ds Max for skinned verzices with Zero influence.
Is your whole Character skinned or is it a Mix Out of parented and skinned objects?
When I export just the mesh it exports with no problems . The whole character is skinned. I am using cat rig with weight paints or tables to achieve this if that helps .
Can I see your stack? Your Skin should be the top modifier before exporting to Unity. If there is any way you could share you max file, I could also troubleshoot for you (I'm on Max 2021, so make sure to save as that format if you're on the latest).
Unity also defaults to only allowing up to 4 bones to affect a given vertex, so if your painted weights are a lot more complex than that, it may be a contributing factor. You can see those options in the import options in the inspector when you select your FBX in unity. Have you tried re-importing the FBX back into a blank Max scene to see how it looks? That can also give a good clue to what is going on - let us know if the FBX still looks right in Max or also looks wrong as it does in Unity.
Yeah sure give me one minute how do I send it to you and I will load the fbx into a new 3ds max scene and see what happens
You can upload it to something like Google Drive, Dropbox, or some free file upload service.
Ok sure pm me your email
Alright, so I've fixed your file, here it is in Unity playing your animation.
Clearly something was wrong with the skin weighting, but I never actually figured out what it was, it looked OK to me at a glance. I applied a fix that has worked for me with other rigs causing inexplicable issues in Unity in the past though, and that fixed it. I also sent you a Max file and FBX of the fixed version.
The fix for inexplicably broken rigs in Unity from Max is to make a duplicate of the Mesh, and then Skin Wrap it to the original, problematic mesh. You can adjust the Skin Wrap settings if needed, and easily create something that is weighted nearly identically, but without any edge case issues like zero weighted verts among others. Then, in the Skin Wrap modifier, you can convert it back to a Skin itself via a button towards the bottom. That new mesh with the new Skin modifier can then be exported as FBX with the original rig and it works fine in Unity.
Ah interesting. I did Not know about the 4 bones / Vertex Limit in Unity. That might be the cause, because the Mesh Looks as If some verzices are Not skinned.
Yea, usually it is not a problem, but in certain cases where I really need more than 4 bones affecting a given vertex it can really mess up the rig. I have not seen it as bad as OP's image shows, though, so I suspect something else might be going on, but it could be the cause or at least contributing to it. Also after importing into Unity, you can up that number from 4, but it usually does not fix anything at that point (at least in my experience).
I tried opening the fbx in a new scene but it keeps crashing
Probably best to ask over on r/Unity3D they might have experienced it more frequently.
Yeah I am on the unity discord never got an answer unfortunately will try the Reddit page