Media Library, some of the plugins, Customize and few more thing doesn’t work

Hi!

From the beginning:
A Few days ago I noticed few lines of code displaying above the header in all pages of the site. I decide to check all the updates and plugins to see what causing the problem. The First thing I did was installing UpdraftPlus to make backup of the site. I made backup on Google, then I upgraded WordPress to 5.7. I also updated Bard Pro theme I’m using.
Then all the problems started. Suddenly I couldn’t log in to the admin site because of the 403 Error, but It only appeared on login page, rest of the site worked good, but with weird code on the top. I left it for a few days, because I needed to contact IT for data to log in the FTP server.
Today I got login data and started working.
First I deactivate all plugins through FTP, then I checked all files permissions to 755 for folders and 644 for files. All site started showing 403 and nothing worked. I deleted .htaccess file, nothing happened, then I changed files to permission a few more times because I didn’t know what else I could do (I waited several minutes after each change to see if anything happen). Finally, the site started working.
And it should be all good, but now nothing works properly on WordPress.
– Media Library doesn’t load
– Customize page doesn’t load
– Pages doesn’t load when I tried to go there from edit panel
– I tried to activate Updraft plugin, and it doesn’t work, all panels where gray and doesn’t respond, so I gave up
All plugins all still deactivated, I tried changing theme to twenty-twenty, but It doesn’t change anything.
Please someone tells me what else I can do, I really need this site to work, but I’m green In all of this and don’t have a clue how this thing works.

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