Hello.
This issue is because of the CSS conflict.
Please make this small fix.
Please open this file:
https://www.sauerlandwillingen.com/wp-content/plugins/booking/css/skins/traditional.css
on line: 37
find this code:
.datepick-one-month { float: left; }
and replace it to this code:
.datepick-one-month { float: left; height: 320px; }

Hello.
You are need to set the number of maximum visitors as 1 for the parent resources at the Booking > Resources page.
Also please note, the parially booked icon in the legend bellow the calendar is belong only the dates, what are booked for the specific time slots, and not belong to the dates with specific availability. You can check the availability for these dates in mouseover tooltip, which you can activate at the General Booking Settings page.

I will try to explain how the system with capacity setting is works, as detail as possible

1) firstly you are need to create the booking resource "test" at the Booking > Resources page.
2) then you are need to create the 2 child resources (during creation of the child resource, you are need to set the parent field as "test" resource) : "test1" and "test2",
So you will have the booking resources structure like this:

test
- test1
- test2

Because you are have 2 child resources and parent resource "test", together it's generate the capacity of the parent resource "test", which is equal to 3 in our example.

3) Now, by default the maximum number of visitors, which is allowed per resource is 1. So we will change it to 2. It's mean that per each resource (apartment), can be maximum 2 visitors.

So we will have the structure like this:

test. |. Max visitors = 2 |. capacity = 3
- test1
- test2

In other words we are have 3 apartment, and each apartment can have maximum 2 visitors per apartment.

4)

Now important options at the general booking settings page:

Visitors number apply to capacity: Check this checkbox if you want that availability of the day (capacity) depends from number of selected visitors [visitors]
Show at tooltip on calendar availability based on free booking resource items.
Check maximum support of visitors at one booking resource with selected number of visitors from booking form:
Show at tooltip on calendar availability based on summ number of visitors, which can be at free booking resource items.
Check maximum support of visitors at all booking resources with selected number of visitors from booking form:

A) If you are do not activate the "Visitors number apply to capacity" so then, during reservation process you will book only ONE available booking resource, do not depend from the any number of visitors selections in the "visitors" select box. So it's mean if you are select 1 or 2 or 5 visitors in the select box at the booking form, you will reserve only one booking resource (firstly its will be parent resource "test", during the next new (second) reservation it's will be child resource "test 1" and during third reservation you will reserve "test 2" booking resource.
So if the "Visitors number apply to capacity" is not checked, the availability for the specific date will be 3 (based on capacity only) and during reservation process the selection number of visitors will not impact to the Availability and just will be saved as a any other field.

B) If you will set checked the "Visitors number apply to capacity" option and activate first radio button in sub settings:

Show at tooltip on calendar availability based on free booking resource items.
Check maximum support of visitors at one booking resource with selected number of visitors from booking form:

so then the Availability in the mouse over tooltip will be based on the capacity of the booking resource. So it's mean that for the free dates the availability will be show as 3, as in our previous example.

But during reservation process, you can not select more than 2 visitors in the select box. Because each booking resource, is support only 2 visitors per resource.

In other words, during reservation process you can reserve only 1 booking resource during one reservation, but the system is also check that the number of visitors, which you are selected in the booking form have to be not more then maximum number of visitors, which you are set at the Booking > Resource page. The availability in mouse over tooltip will be based on the capacity and in out example will be show as 3

C) and now last variant if you are selected the bottom radios button option:

"Show at tooltip on calendar availability based on sum number of visitors, which can be at free booking resource items.
Check maximum support of visitors at all booking resources with selected number of visitors from booking form"

So it's mean that availability over the specific date will be based on the capacity and number of maximum visitors per resource. In our example it's will be show as 6 ( 3*2=6).
During reservation process the system can book several booking resources, it's will calculate the visitors, which you are select in the booking form for the each booking resource (parent or child) and number of visitors per this resource.

So for example if you are select 3 visitors in the booking form. It's will reserve 2 booking resources. Because first resource is support 2 visitors and second is support also 2 but we are have only 1 (3-2=1), in other words one bed will be free in the second resource.

So hope it's clear now.