Can't remediate ESXi with VUM

Hi,

I have VUM 5.1 u3 and I can't update one ESXi with it. ESXi version is 5.1 u2. When I run remediate task it fails after some time with an error that VUM was not able to put host into maintenance mode. If I put this host into maintenance mode manualy, VUM is able to install required updates. So how can I fix this situation? Thanks.