If remediation of any host patches requires the host to be put into Maintenance Mode, VMware vCenter Update Manager invokes the Maintenance Mode task to do the migration for it.
However, with Update Manager 4.0, the remediation fails when it is initiated against a host running vCenter Server or Update Manager contained in a virtual machine. There is no attempt to migrate those virtual machines. This is expected behavior.
To workaround this issue, manually migrate the vCenter Server or Update Manager virtual machine off of the ESX host that you want to remediate.