Very often encounter this error. Any tips, for example, to remove restrictions, edit the .htaccess file didn't help. The same applied for help in hosting tech support, but even there have not been able to solve this problem.
It turned out that 30 of the installed plugins, 2 "conflicts", which when uploading new images, clip art, HTTP error and error Internal Server Error. One of the plugins was Regenerate Thumbnails. It is due to these čërtovyh plugins are not loaded image on the site.
So out of this situation: deactivate all plugins, find conflicting and deactivate it. You can deactivate a plugin, then 3 check-image upload. Cheers, I have everything working.
Read more here https://chempalov.ru/?p=17226