Going back to an older version of XO source also did the trick for me.
I was in the process of pushing in production a couple VM that were previously in test and that I had backed up a couple of days ago. I was very surprised not to be able to do anything with these VMs stuck on my SR used for VM backup

@tjkreidl

The command "xe task-cancel force-true ..." with the uuid of the pending tasks did not do anything for me. After trying it on the 3 tasks I had, issuing a "xe task-list" still show me the pending tasks. It it only by rebooting the host that the pending tasks disappear.