be carefull to update your WP 4.9 to 4.9.1.
the plugin won´t work under 4.9.1 so far.
if you have updated your wp you can download the version 4.9 here: https://wordpress.org/download/release-archive/
or you can download the plugin WP Downgrade to get the WP 4.9
Thats not true, we are running V4.9.1 as well and do not have any issue. If you have any problem please contact the support by creating a ticket.