As an alternative, you can also use the extended SSH service to see the disk usage:
ssh -p23 uXXXXX@uXXXXX.your-storagebox.de df -h
Filesystem Size Used Avail Capacity Mounted on
uXXXXX 100G 17M 100G 0% /home
You can use the argument -m to get the output in megabytes:
ssh -p23 uXXXXX@uXXXXX.your-storagebox.de df -m
Filesystem 1M-blocks Used Avail Capacity Mounted on
uXXXXX 102400 0 102399 0% /home