I followed more or less the example "Special and Read-only Slots"
in order to achieve the desired behavior (show working hours in a different color).
As you can see in the first screen shot (sched01.png), everything works fine.
As soon as I use custom grouping, the working hours are no more visible (see sched02.png).
After deselecting the group item (here: Zimmer 315), the working hours for the selected person are displayed again.
Your help is highly appreciated.