Bookings calendar blocking timeslots that are available

Appointments cannot be made for some calendars in Bookings, although there are available timeslots in those calendars.
This problem has surfaced since this week.
Appointments can be scheduled again for all agendas in the Bookings calendar from 31/1.
The times do not match our opening hours every day. Looks like a timezone error, but only for 6/1 and not the other days.