This is the type of error:

2015-11-12 16:07:26+0100 [ClusterClient,client] Failed to refresh power state.
        Traceback (most recent call last):
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 423, in errback
            self._startRunCallbacks(fail)
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 490, in _startRunCallbacks
            self._runCallbacks()
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 577, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1155, in gotResult
            _inlineCallbacks(r, g, deferred)
        --- <exception caught here> ---
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1097, in _inlineCallbacks
            result = result.throwExceptionIntoGenerator(g)
          File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
            return g.throw(self.type, self.value, self.tb)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/power/query.py", line 126, in get_power_state
            system_id, hostname, power_type, context)
          File "/usr/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1097, in _inlineCallbacks
            result = result.throwExceptionIntoGenerator(g)
          File "/usr/lib/python2.7/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
            return g.throw(self.type, self.value, self.tb)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/drivers/power/__init__.py", line 246, in query
            self.power_query, system_id, **kwargs)
          File "/usr/lib/python2.7/dist-packages/twisted/python/threadpool.py", line 191, in _worker
            result = context.call(ctx, function, *args, **kwargs)
          File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 118, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 81, in callWithContext
            return func(*args,**kw)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/drivers/power/vmware.py", line 70, in power_query
            host, username, password, vm_name, uuid, port, protocol)
          File "/usr/lib/python2.7/dist-packages/provisioningserver/drivers/hardware/vmware.py", line 480, in power_query_vmware
            .format(uuid=uuid), traceback.format_exc())
        provisioningserver.drivers.hardware.vmware.VMwareAPIException: (u'Failed to get power state for uuid=', 'Traceback (most recent call last):\n  File "/usr/lib/python2.7/dist-packages/provisioningserver/drivers/hardware/vmware.py", line 472, in power_query_vmware\n    vm = _find_vm_by_uuid_or_name(api, uuid, vm_name)\n  File "/usr/lib/python2.7/dist-packages/provisioningserver/drivers/hardware/vmware.py", line 428, in _find_vm_by_uuid_or_name\n    vm = api.find_vm_by_name(vm_name)\n  File "/usr/lib/python2.7/dist-packages/provisioningserver/drivers/hardware/vmware.py", line 238, in find_vm_by_name\n    if vm_name == vm.summary.config.name:\nAttributeError: \'vim.Folder\' object has no attribute \'summary\'\n')