empty page at admin calendar

Hi there,

I have the appointment calendar premium. Since a few days (I am not sure when) I have an empty page when clicking in the admin on appointment calendar > admin calender.

Turning off all other plugins does not help.
When switching to a deafault theme I get this error:
Fatal error: Cannot redeclare class Error in [sitename]/httpdocs/wordpress/wp-content/plugins/appointment-calendar-premium/menu-pages/settings/google-api-php-client/src/contrib/apiCalendarService.php on line 1081

Please help me solve this.
Thanks

https://ww.wp.xz.cn/plugins/appointment-calendar/