Unable to choose "Custom Hours" in Outlook Bookings drop down-menu

I've created a shared booking calendar in Outlook Bookings and I am able to add regular time slots for the different services. However, when I'm trying to choose the option "Custom Hours" in the drop down-menu, it automaticly changes to the option "not bookable". About a month ago I didn't have this problem, but during the past few weeks it has been impossible to add custom hours. Can you please help me with this problem?