Spam/Hack on top of source code! (RSS Feed wont work anymore)

I am having quite the problem with my two blogs recently where someone has inserted some kind of wrong tag into the very top of everyone of my WordPress pages.

You can view my site here (https://www.meandisis.com/) and right-click “View Source”. Look at the top, you will find: “<!– test1 –>” just before the Doc Type. That is on EVERY single WordPress page, even in the WordPress Admin files.

How can this be fixed? We looked into the database to find the code, replaced all files with brand new same version WordPress files, disabled all plugins and reactivated again and we couldn’t find anything positive related to this “test1” thing, it won’t go away…

I would really appreciate any heads-up.

Thanks a lot!