I have three VM hosts which running ESXi 5.1, 799733 and then I upgraded two hosts to ESXi 5.5, 1746018. After all, I found some VM cannot be migrated from ESXi 5.1 host to ESXi 5.5 host.
Live migration, vcenter will show "The operation is not allowed in the currecnt state".
If I power off the VM and migrate it, it can complete the migration successfully however the VM will become "VM_name(invalid)". Then I need remove it form inventory and register it to ESXi 5.1 host again.
Just some VM has this problem. I have try to live migrate some VM from ESXi 5.1 host to ESXi 5.5 host without any problem. So I can confirm the VMotion network between these three host is not a problem.