On a couple of my WordPress sites, I’ve encountered an HTTP Error when trying to upload media. I tried many solutions: increasing PHP’s memory allocation, changing file permissions, using GD instead of Imagick, adding lines to my functions.php, etc. None of them worked.
But I found a workaround. A simple plugin called Add From Server allows you to add media to that is already uploaded to your website’s server. So, all I need to do is upload the files to my server, add the media from there, and go about my business.
It’s not a true solution, only a workaround, but it’s a more-than-adequate workaround until WordPress can find a singular solution to this elusive error.
Update: Moving from a Debian 7 to Debian 9 VPS seems to have eliminated this problem.