In the morning I was trying to remove snapshot from VM using vSphere client, to my surprise, I couldn’t remove it.  An error says:

Remove snapshot <VM name> Another task is already  in progress. < domain\user> <virtual machine name> console

I tried to stop VM, without success, error this same like above.

Second try-out, from command line using vmware-cmd command with removesnpahots syntax, failed with error:

VMControl error -999: Unknown error: SoapError: ServerFaultCode(0) : (Operation failed since another task is in progress.)

I  solved a problem by restarting vmware management agent on ESX host, # service mgmt-vmware restart. After process restart I was able remove snapshots and power VM down