Thanks to all for posting your solutions. We recently ran into this exact problem on two new VPS servers at Inmotion. In our case, the solution was the same as above, adding one line to the .htaccess file in /public_html/
Add to .htaccess:
SetEnv MAGICK_THREAD_LIMIT 1
Thank you for sharing a solution skam1922. The first update to the plugin will likely erase that modified code, however.
Working on a more permanent solution which I will post here if successful.
