Activation teamstuff plugin gives errors

Hi there,

I have 2 environments: in my test environment (https://www.ucetk.org) the plugin works, in my production environment (https://www.wasps.nl) I get errors when activating the plugin:
In the plugin console I get:
Teamstuff_Calendar_Admin::$option_name in /htdocs/public/www/wp-content/plugins/teamstuff-calendar/admin/class-teamstuff-calendar-admin.php on line 145 Notice: Undefined property: Teamstuff_Calendar_Admin::$option_name in /htdocs/public/www/wp-content/plugins/teamstuff-calendar/admin/class-teamstuff-calendar-admin.php on line 154 Notice: Undefined property: Teamstuff_Calendar_Admin::$option_name in /htdocs/public/www/wp-content/plugins/teamstuff-calendar/admin/class-teamstuff-calendar-admin.php on line 163

Both environments are identical, so I am completely baffled. Any clues?

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