Yes sure,
It was just something I did wrong.
If you want to reduce /dev/pve/data to 10GB, the following will work without any problem:
umount /var/lib/vz
e2fsck -f /dev/pve/data
resize2fs /dev/pve/data 10G
lvreduce -L 10G /dev/pve/data
resize2fs /dev/pve/data
mount /dev/pve/data
Note: If...