Recently I have came across this annoying error: Identity source cannot be added in vsphere Web client interface via SSO configuration to add Active Directory to be my Identity Source for Users authentication in my vSphere 5.1 Platform.
The issue is Very straight-forward if you have read the Error description with patience; Authentication issue: Provided Credentials are not valid error. However, the Administrator account seems to be accurate with the password, but still throws this error. IN such cases try the below steps to isolate the issue straight away.
1- Make sure the Administrator Password is Correct and it hasn’t changed since last known.
2- Make sure the Administrator account is a member of Domain Administrator account: Will be default, if Default Admin account is being used.
3- If everything is failing, Try using a different Account, whereby promoting/creating a Domain Admin Account and providing the same.
Once the valid credentials are provided, Identity source can be added without fail this time.
You may also encounter, LDAPS security error, where in The Domain Environment, the rejection of normal LDAP protocol is set via Group Policies. In such cases, you should use LDAPS protocol to connect to AD Source and use the certificate of LDAP security. See the below links about the LDAPS error.
Video: How to add Identity Source in vSphere 5.1 via Webclient