We recently upgraded our hosts from ESX classic to ESXi 6.0. Since then our AD users are unable to login to the host nor are we able to assign new AD users permissions.
The hosts are joined to the domain and I've tried leaving & rejoining (both which work fine).
The errors are,
Cannot complete login due to an incorrect username or password or the following names were not found and could not be added.
Are the users using FQDN when they are logging in?
ex, "firstname.lastname@example.org" or are they using "domain\user" or simply just their username?
1. Can you try to login with AD user/pass in server where you installed vCenter and see if they authenticate with AD
2. Ping from ESXI host to AD domain with name instead ip address and see whether it is reachable
3. provide any screenshot for better understanding with situation
Also, check here,
vSphere Client under "Host -> Configuration -> Authentication Services -> and change Directory Service Type fron Local Authentication to Active Directory.
then you just add AD Users/Groups under permissions