When trying to prepare a host (after an unprepare) in vCloud Director 8.20 (ESXi 6.0) I got the following error:
Remote call failure on host [vcId=8af60e1e-3f9d-4d01-a69b-0f4c94e7f09a, moref=host-590541]. Error string was " A general system error occurred: Weak password: not enough different characters or classes. Please refer to the log for more details.".
This appears to be a bug. Simply reboot the ESXi host and then prepare the host again.