Check the error log of your server, software like mod_evasive create such error with upload progress bar as progress bar do lot of requests to server to update the stats, mod_evasive disable access thinking it is done type of attack. Try disabling upload progress bar and see if upload works.

include/settings/upload.php

Code:
$use_upload_progress_bar = 1;
Set it to 0

Code:
$use_upload_progress_bar = 0;