This might not be it but could the file path have something that might get filtered? If not, are you uploading it and renaming it as something that might be filtered?
Sorry! Poor wording on my part! Just use a privative from your choice of 3d software, Privatives are the basic shapes like a sphere. So just try uploading a cube to see if it uploads.
I agree with Nick on the age restriction as I had to create an alt account to be able to upload meshes. Also, if it fails to upload too many times due to the strange name error( which seems to be the default error back) then you will be blocked from uploading new meshes for a period of time.
I read somewhere to rename your mesh to “dog” before upload to make sure that you don’t get hit by the name error, but not sure how true that is.