This may be pertinent for WordPress installs on InMotion web hosting only.
I ran into an image upload error for a new WordPress install where the only message displayed was “HTTP error”. After following the usual actions of checking folder permissions, php memory limits, etc. the solution that worked for me was found in this post from InMotion: How to Fix the HTTP Image Upload Error in WordPress.
Just edit your .htaccess file in the root of your WordPress install (mine was located at /public_html) and add this line to the top of your htaccess file:
SetEnv MAGICK_THREAD_LIMIT 1
I saved the changes and the media uploader jumped to life and began working as it should!