it sure looks like you're either not provisioning more then 100GB of space (which you're provisioning to root) or you're not using the added space.
check when your data share is mapped to. I'm guessing its your "local" share, mapped to /var/lib/vz. Since its sitting atop of root file system you just ran out of space. if you selected the lvm partition type for install, /var/lib/vz is remapped to a seperate partition. however, if you selected another type (eg zfs mirror) this is actually mapped to the root file system.
The solution:
1. create a partition, LVM, or any storage object on the remainder of the disk.
2. add it to your storage repository (Datacenter -> Storage -> Add)
3. migrate or delete everything in /var/lib/vz
4. disable the "local" dataset (since proxmox doesnt allow you to delete it.)
user data mounts should never be allowed on a root file system. To the devs- why is the local dataset created at all at install? this can and should be set up by the administtator based on their storage plan.