I'm running Multisite 3.6 currently and am getting an HTTP error when I try to upload a featured image in to a custom post type using an editor role. There are many factors involved in this installation, so I'll try to give the best description of my site as well as what I've done to troubleshoot it already.
First off, the problem began when I was running 3.52, it originally worked okay, but about a month ago all I could get was the HTTP error. I have editor roles attached to each individual site, and they have limited access. I've used the Admin Menu Editor Pro plugin to do this. Basically I the editors should only have access to two different post types, and only one type has a featured image. It should also be noted that there are around 130 sites involved in this installation, and that this problem appears to show up in every role except for Administrator.
When I try to upload, it takes me to the regular post upload box, but the Media Library section is empty even though there are images on the site. I can go to the Upload section and choose a file, but then I get the "HTTP error" print out on the right side of the popup box.
In any case, I made a copy of the site on another server, updated WordPress, updated the theme and updated all of the out of date plugins. I wanted to test all of this out before I did it to the live site anyway. Everything was fine with the updates, but still this problem persisted.
Then I disabled all of the plugins, including the Admin Menu plugin hoping something would change. Still, persisted.
In the functions file I added the capability of 'upload_file' to the editor role thinking it might be a permissions problem. Still I get an error, but this time I get "An error occurred in the upload. Please try again later." as the error instead of the more generic "HTTP error"
I've also run the WP_DEBUG option in the config file and found nothing but a Notice having to do with has_cap being depreciated. I'm not sure where this is used, but since it's only a notice I don't think it's what's causing the problem either.
I'm kind of at a loss as to what could be causing this. Does any one have any ideas or suggestions?