I had to restore about 20 websites today because of this issue. I did the following to fix:
Restored backups (Still ran into issues)
Restored entire server (still ran into issues)
Edited index file and removed script
and updated all plugins on every account
The plugins that I ran into that were causing it were:
Really Simple SSL, WP File Manager & Squirrly SEO… Please update these plugins!!!!
That seemed to fix it for now