How do you know this?
Because of df -h and RPI-Monitor. The latter shows 4.71 GB free on / at the moment and df -h this:
Dateisystem Größe Benutzt Verf. Verw% Eingehängt auf
/dev/root 59G 54G 2,3G 96% /
devtmpfs 236M 0 236M 0% /dev
tmpfs 241M 1,3M 239M 1% /dev/shm
tmpfs 241M 6,5M 234M 3% /run
tmpfs 5,0M 4,0K 5,0M 1% /run/lock
tmpfs 241M 0 241M 0% /sys/fs/cgroup
/dev/mmcblk0p1 43M 23M 20M 53% /boot
/dev/sda1 7,5G 3,7G 3,9G 49% /media/extern
tmpfs 49M 0 49M 0% /run/user/1000
tmpfs 49M 0 49M 0% /run/user/998
In wonder why there is a discrepancy between both tools which was not the case when I once compared both. It was some weeks ago, though.
Just now, when I writing this, the Pihole ran fine since I posted the question this afternoon. I have just opened the section "Tools" in the web interface and it suddenly showed this "undefined%" message and deleted the graphs. Nothing is written in the "Pi-Hole diagnosis" section.