Warning error in formatting.php, line 2728

I have this warning on one of my site pages (https://alabuzz.net/client-support-just-give-me-a-ding/):

Warning: mb_substr() expects parameter 3 to be long, string given in /home/joanne4/public_html/wp-includes/formatting.php on line 2728

Warning: mb_substr() expects parameter 3 to be long, string given in /home/joanne4/public_html/wp-includes/formatting.php on line 2728

Yes, the warning is repeated twice.

I have no idea what to do with Line 2728. I did try re-installing that particular WP 3.2.1 file, but it didn’t help. Deactivated all plug-ins, but didn’t help.

Please, can anyone help? Thank you!