The bottom bar indicated a duplication that I canceled several times. This stopped the duplication until a reboot, but didn't help me to disable duplication. I also performed re-measures, which also did not help.
However, my problem just got resolved, because the tree reappeared. Interestlingy suddenly a green "2x" label appeared next to the pie chart and the "Pool file duplication.." gave me the option to disable the duplication instead of enabling it.
Seems that somehow I got into a state where Drivepool already duplicated the data, while the UI part of the application did not know that duplication was activated.