VMControl error -999: Unknown error: SoapError: ServerFaultCode : (Operation failed since another task is in progress.)

Situation. You want to power off VM, but vc is sending message that operation failed since another task is in progress.
You start to wonder, WHAT TASK!?!?!?!?, in tasks you can not see any tasks that are ongoing. So where is that task ?
Login to esx via ssh for example.
[root@gxxx uuu# vmware-vim-cmd vimsvc/task_list
(ManagedObjectReference) [
‘vim.Task:haTask-910416-vim.VirtualMachine.createSnapshot-153439’,

There you go 😉 in my case it was due to last night backups. They have started to “create snapshot” which has timed out. Apparently that task was still there and blocking other tasks.

Solution:
in my case restarting service console helped: service mgmt-vmware restart.

Useful informations that might help :

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1013003