Usually this error is seen when something is changed to the vpxuser user account, as also described in VMware's KB2017460.
The solution to the problem is pretty straight forward, you disconnect the host from vCenter and use the DCUI or a ssh session to remove the vpxuser user account. After you re-connect the host again to vCenter, you ignore the warning about the bad user name and password and enter the root credentials of the host when prompted. The host will now be re-connected and vCenter will create a new vpxuser user account. After this process is finished the host should be as it was before the issue.
In this particular case I found something strange, of the 8 hosts with this issue 7 hosts did not have a vpxuser account at all. When I tried to delete the user account it just reported back "unknown" only the 8th host (I know it's always the last one) did have a vpxuser user account.
The important part to know is that when there is no vpxuser user account just a disconnect followed by a re-connect will solve the issue.
