I had the problem with unstoppable tasks a couple of times recently and found no other way to clean this tasks but restarting the VCenter services. I found out that a possible reason for the the problem are long running tasks of single VMs during the phase where the backup snapshot is being removed. Some servers under heavy I/O can take up to one hour until this is done (since the filer serving the disks is not under load, it seems to be a problem of the VM) and I think a timeout in the whole backup process happens somewhere. The strange thing is that whenever I have a hanging backup job, the backup snapshot on the storage side (volume snapshot) is not done. In my understanding this snapshot should be triggered after the last VM snapshot has been done but before they are removed. As I workaround I tried to set different backup windows for the jobs and so far it worked.
