Warning message in formatting.php

Hi,

I have just moved my blog to a new URL.
Everything is fully OK, working and all, but if I go into any of my posts, I have a warning message at the beggining right before the body of the text saying: Warning: strip_tags() expects parameter 1 to be string, array given in (root, changed it)/html/wp-includes/formatting.php on line 631

Now line 631 is the following:

629function sanitize_user( $username, $strict = false ) {
630 $raw_username = $username;
631 $username = strip_tags($username);
632 // Kill octets
633 $username = preg_replace(‘|%([a-fA-F0-9][a-fA-F0-9])|’, ”, $username);
634 $username = preg_replace(‘/&.+?;/’, ”, $username); // Kill entities

Could anyone help me with this please, I am really lost now.
Is it a plugin that is not working anymore?
I have only a couple, like : exclude pages from navigation, subscribe2, WP post column and YARPP

Or is it something completely different
Thank you