'Theme support for post-thumbnails stopped working after upgrading to Wordpress 5.7

I just upgraded to Wordpress 5.7 and I don't see the option to add a post thumbnail any more.

I checked my functions.php and everything is still fine:

add_theme_support( 'post-thumbnails' );

I have tried all kind of stuff, but nothing that solves the problem, where it used to work perfectly before the upgrade.

I also tried:

function theme_setup(){
    /** post thumbnail **/
    add_theme_support( 'post-thumbnails' );
}
add_action('after_setup_theme','theme_setup');

Any ideas?

Solution 1:^[1]

Here's the solution:

It was because of an older version of jquery-ui.

Make sure you have the index.php file exist.
See my answer https://stackoverflow.com/a/71261080/5413283

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Solution	Source
Solution 1	user3004118
Solution 2	Dexter