Unable to export forms

Using VFB Pro Classic, version 2.5.1. Export of forms fails with browser error
Firefox can't find the file at http://test.pushingouttheboat.co.uk/wp-admin/admin.php?page=vfb-export
and the following fatal error in the WordPress debug.log file
PHP Fatal error: Call to a member function get() on a non-object in /var/www/web/wp-includes/query.php on line 27

The export fails with these errors when either the option ‘All’ is checked or the ‘Forms’ option is checked but works OK if the ‘Entries’ option is used. (Which is not much help since I want to export a form developed in my test system and import it into the live system.)

I last used the Export function successfully back in September, after I upgraded VFB from 2.5 to 2.5.1 but before I upgraded WordPress to v4.3.1 so I’m guessing that it may be the WordPress update that triggered the problem.

Have you any ideas what is causing this and how to fix it?

https://ww.wp.xz.cn/plugins/visual-form-builder/