To fix this, in most cases it is advisable to edit the .htaccess file to get rid of it, as it is the simplest solution in such cases.
- Log into cPanel and access the file manager / Connect via FTP to the account having the issue
- Find the .htaccess file in the directory where you have installed Wordpress.
- Edit the file, adding the following line at the beginning of the file: SetEnv MAGICK_THREAD_LIMIT 1
- Save the file and see if your problem has been fixed.