Cannot redeclare wp_slash_strings_only() Critical Error after updating to 5.6

Hi champs

Recently i’ve updated WP to 5.6 core and plugins. After getting a Critical Error, i’ve updated the themes via FTP.

But the error is still there, after clearing cache and cookies.

I’ve disables plugins by renaming the plugin file names in plugins folder.

I’ve enabled log errors. I got this:

Cannot redeclare wp_slash_strings_only() (previously declared in /home/##/public_html/wp-includes/formatting.php:5559) in /home/##/public_html/wp-includes/deprecated.php on line 4120

I’ve checked the code at the two locations in the error. But i’m a roockie!

Is there any idea on this? Is it a WP 5.6 bug or error? Or am i missing something?

Cheers in advance.
The best.

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