Warning: join(): Invalid arguments passed in post-template.php

You did not returning the default class if you are not viewing the contact page. Try this code once

if(! function_exists( 'remove_hentry' )) {
    function remove_hentry( $classes ) {
        $classes[] = customTheme_layout_class();
        if( is_page('contact') ) {            
            $classes = array_diff( $classes, array( "hentry" ) );
        }
        return $classes;
    }

    add_filter( 'post_class', 'remove_hentry' );
}