did you lost connectivity with that datastore1 ? Please validate that.
Also you can detached that disk from that VM and check it on another working VM if it's get full then you need to make free space on this disk.
You need to stop all activty on this datastore at once !!!
Every further attempt to do anything on this datastore will either fail - if you are lucky - or make matters worse.
The path /vmfs/devices/deltadisks/... has links to the delta.vmdks that are currently in use: either by a active VM or by a stale lock caused by a misbehaving automatic backup tool.
Next thing to do: stop all VMs that use thin provisioning or snapshots and free up some space.