When attempting to join your vCenter appliance to AD, you may get the following error message when clicking the Join button:
This is known behaviour when running vCenter with an external Platform Service Controller (in HA mode) but the issue may persist even after upgrading vCenter to a new version with the option to switch to an embedded PSC. You can work around the issue by performing the domain join from the CLI (through SSH):
/opt/likewise/bin/domainjoin-cli join [domain] [username]