I’ve spent HOURS trying to find a long-term solution while waiting for WordPress to update.
The image upload will work for a couple days then sporadically stop working, and eventually stop working altogether.
I disabled all plugins and switched to the 2016 theme. Works like a charm every time. If I switch to my custom theme it’ll eventually (after a few days) stop working again.
I’m on a shared server so I can’t update the Imagick module as WP suggests. I know that’s not the ultimate solution, but that works for now.
I’d also like to note that all the functions.php fix (as facter mentioned) and other htaccess edits didn’t work for me either… theme switching was the only crappy solution.
Hope that helps other people who cannot update the Imagick module! ??