Fatal error: Cannot redeclare sanitize_url()

Hello,

After searching for a solution for several hours in vain, I am posting a help message.

I am getting this error message on the website linked to this post: Fatal error: Cannot redeclare sanitize_url(). I only have access to the server files.
I tried deactivating the plugins, then the theme, but nothing changed.
I also wanted to update WordPress manually, but I don’t know how to make a backup of my current WordPress version containing my theme settings (maybe not possible?).
I am not sure what to do when a function is declared twice (that’s the only answer I found about my issue).

Thanks for your help.

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