Hello again,
I was able to solve the problem by myself.
The cause was as follows:
During the installation procedure, MaxDB Server Software was installed, but not correctly. But the Database Instance was already registered. So the SWPM didn't know, what to do as there is a database instance but it doesn't work correctly.
The solution was to deregister the MaxDB instance by dbmcli-command. The dbmcli program is located in /sapdb/SID/db/bin. The command to deregister the instance is called db_unreg.
After the deregistration of the database instance I was able to deinstall all system components and install the whole SAP system again. This time it worked fine.