Whenever I try to upload data then after going half process it gives an error like attach an image.
Thanks for asking your question at Envato forum.
I have face this problem.
Clean your browser cache & restart your browser then try again hope will uploaded.
Try With clearing browser cache but not help and getting same error.
you can try with other browser or make a restart your Computer.
also try in different computer getting same problem.
Then you should opt in for FTP upload instead, it is the most certain way of uploading.
Use: ftp.marketplace.envato.com as host
Generate a legacy token in your profile, and use as password.
Once you upload via ftp using client like FileZilla, just refresh the item submission page and you will see you files there.