I got this error by 2 week and i do not know whats going on.
If someone knows pleas comment on this topic.
this error happens when i truing upload motion graphics item.
you can try with Envato FTP upload system hope your issue will solved.
How to upload using FTP
Stock Footage & Motion Graphics Uploader
Also this topic will helpful for you.
I think the motion graphics uploading becomes restricted, you have to fill the form and then you can use it after the reviewers checking your items quality, check this out here