Responsive Menu WP Plugin: Uncaught Error: Object of class WP_Error could not be converted to string

I transferred my site to a laravel server and since then I'm having trouble with the responsive menu. This is my error:

Fatal error: Uncaught Error: Object of class WP_Error could not be converted to string

in this code of my plugin:

1.  $css = '';
2.      foreach ( $menu_ids as $menu_id ) {
3.          $css .= $this->get_css_for_menu( $menu_id );
4.      }

The error happens in line 3. In the previous environment it worked fine.

I tried to reinstall the Plugin and Update, switched PHP Versions from 7.4 to 8.0 to 8.1 and back, nothing worked.