Move into resource pool Cannot complete operation due to concurrent modification by another operation

Hi Everyone

I have three hosts on my vcenter, however I can't seem to move any VMs into a resource pool as I get the following error(for example):

Move into resource pool SALES Cannot complete operation due to concurrent modification by another operation.

Any ideas? I can move VMs around on the other two hosts without a problem. The only difference is that the problematic host is running ESXi version 5.1.0 1065491, the others are running 5.1.0 799733

Any help would be greatly appreciated...

Thanks!