Random letters appearing on all pages

Hello,

I’ve just switched from http to https, and I have the weirdest error :
Three letters showing up at the start of each and every page of the website “ser”.
I’ve deactivated all the plugins, and all the themes, but the letters kept showing up.
I’ve inspected the page, and it looks like those three letters are showing up right after the body tag.
Is there a way to find the cause of these letters showing up ?

Thank you

The page I need help with: [log in to see the link]