Media Upload HTTP Error

Good Afternoon.

We host hundreds of WordPress sites, and have come across our share of issues that are normally able to be resolved. However, we’re now having issues with uploading Media – and so far we’ve been unable to fix the issue itself. I’m certain I’m missing something so simple here, and that I’ve merely clouded my mind up. But I certainly hope someone can help me to resolve this issue, and such would be greatly appreciated.

First, the background. When trying to upload media on the site, we receive a simple “Http Error”. I began by trying all the regular stuff:
-Reinstall WordPress Core Automatically.
-Reinstall WordPress Core Manually.
-Deactivate All plugins (There are only 1, Shareaholic)
-The Site is currently running the twenty thirteen theme, with minor customization.
-Check file/group permissions and verified they were correct.
-Cleared out server’s /tmp files, and removed all duplicate media and thumbnails.
-Numerous other minor attempts, including searching the WordPress database and putting in fixes from previous versions.

That being said, nothing has changed server side, the only change was the auto-update to 3.8.1, which seems to have since broken the media uploader (It was working fine before). The memory limits, upload size, and associated issues are all extremely high, if not unlimited. So that is not the issue either. You can see the php confirguation at https://beaconnet.com/phpinfo

This site is one of about fifty or so WordPress sites on this server, and the only one experiencing this issue until yesterday, when a second site began experiencing the issue for no reason. They are on the same version, and have been operating normally now under it for some time.

I did notice that files under 500×500 can be uploaded, but anything larger fails. Adjusting the sizes in the media settings doesn’t help, but if I update the thumbnail size to say, 700×700, it will actually work.

I’m confusseled, to say the least, and would appreciate any help as quickly as possible! Any questions, let me know!