Would you mind giving a short feedback on what went wrong on your end? I have a weird situation where I usually get an UPID, .. but SOMETIMES I do get an empty string, but the task was still successfully queued by Proxmox. The problem is that in this case I never "know" when the task is finished or if it was started at all.
I wonder if there is some sort of edge case in which a task was started but does not return an UPID ...