Media Uploader HTTP issue

I know there are a lot of posts about this issue, but most of them are 2+ years old. I know that the recent release of WP 4.4.2 brought a lot of changes, so I am posting about this.

This site I built is supposed to show thumbnails on some pages. Not only are they not displaying, we’re also getting an HTTP error when uploading images. But even though it displays an HTTP error, the images are still uploaded. It looks as though it’s not generating the thumbnail versions?

I’ve uploaded a screenshot.

Also, when I try to use the old browse uploader, it returns a 404 Error when I click “upload” (but again, still uploads the file anyway).

Here are things I’ve done that haven’t worked:

• I increased the upload limit to 64MB in wp-config
• I modified the permissions by adding that “IfModule mod_security.c” code in .htaccess
• Updated all the plugins, themes, etc.
• Reinstalled WP (via the Dashboard)
• Logged out, cleared browser cache and cookies, logged back in
• Disabled all plugins
• Tried a variety of sizes, images, screenshots, photos.

This is happening for me and my friend (whose site it is). So I don’t think it’s computer/browser specific.