Ticket #54987: 54987.patch

src/wp-includes/user.php

@@ -1995 +1995 @@
          */
         $user_nicename = apply_filters( 'pre_user_nicename', $user_nicename );
 
+        if (mb_strlen( $user_nicename ) > 50) {
+                $user_nicename = mb_substr( $user_nicename, 0, 50 );
+        }
+
         $user_nicename_check = $wpdb->get_var( $wpdb->prepare( "SELECT ID FROM $wpdb->users WHERE user_nicename = %s AND user_login != %s LIMIT 1", $user_nicename, $user_login ) );
 
         if ( $user_nicename_check ) {