A PHP 8.0 peculiarity

Setup:
Fresh website, with GeneratePress.

Problem:
It all works fine with PHP 7.4

It also works fine with PHP 8.0, but I get this error in the WP root directory (on the server):

[15-Sep-2021 19:59:44 UTC] PHP Warning: Undefined variable $custom_content in /home/blablabla/public_html/wp-content/themes/generatepress-child/functions.php on line 26

Here's what's in line 26 (the bolded line is 26):

// BEGIN LAST-EDITED-updated ISPISIVANJE

function wpb_last_updated_date( $content ) {
    $u_time = get_the_time('U'); 
    $u_modified_time = get_the_modified_time('U');
    if ($u_modified_time >= $u_time + 86400) { 
        $updated_date = get_the_modified_time('d/m/Y');
// THE PROBLEMATIC LINE 26:
        $custom_content .= '<p class="last-updated">Updated: '. $updated_date .   '.  </p>';
    }
    $custom_content .= $content;
    return $custom_content;
}
add_filter( 'the_content', 'wpb_last_updated_date' );

// END LAST-EDITED-updated ISPISIVANJE

The function does its job with no problems, even when that error gets reported in the root directory.

When I install and activate a plugin called "Subscribe To Comments Reloaded," the error code goes away.

Likewise, if I don't change anything, just set the PHP back to 7.4 - the problem goes away also.

I've checked everything else - PHP extensions, configuration, asked the provider to confirm the 8.0 is running OK.

I use the above-noted plugin on several websites - and they had no problems with the error.log appearing.
That's how I finally figured out the... strange situation.

The problem happens with GeneratePress and with GeneratePress premium.
It doesn't happen with TwentyTwentyone.