We see an issue like this once and a while, and at least in our environment it is always the same thing. The DVS has used all the ports up on a given host, and just can't make the connection.
We had to change (increase) as setting on each host. "Networking->DVS->switchX->"Properties" -> "Maximum number of ports designated on this host"
The default is 256, we had to increase to 1024 in order to stop running out of ports.
Hope this helps.