iCal Calendar Events Link Broken

I run Kanboard in a subdirectory of my web server with nginx. The url reads like: https://xxx/kanboard/
I subscribe to my iCal feed: https://xxx/kanboard/?controller=ICalendarController&action=user&token=yyyy
I can easily subscribe to the calendar with Apple Calendar and Thunderbird. All Events are displayed fine.
But the link form there to the task in Kanboard is broken. It reads as follows: https://xxx/kanboard//kanboard/?controller=TaskViewController&action=show&task_id=111project_id=1. The part /kanboard/ is doubled.
In “Application settings” I have set https://xxx/kanboard/ as the “Application URL”

With RSS feed there is no such a problem.

Is there anything I can change in the settings or configuration?

Thanks in advance