vCenter 5.1 unable to login using vsphere client

Issue: When user tries to log into vcenter 5.1 using vsphere client by checking use windows credentials it fails with errors.  It was a mixed environment with 1 vcenter 5.1 & 4.1
"Cannot complete login due to an incorrect username or password"

what didnt work : 

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2035510

1. Log into the vSphere Web Client using the Admin@System-Domain credentials.
2. Click Administration.
3. Under Sign-On and Discovery, click Configuration.
4. Click the Active Directory identity source.
5. Under Actions, click Edit Identity Source.
6. Make note of the information in the identity source.
7. Click Cancel.
8. Under Actions, click Delete Identity Source.
9. Recreate the identity source using the short NETBIOS name in the Domain Alias field.
10. Click Test Connection.
11. Click OK. 

Tried all the above but no go.
 now another error started coming up
VMware Infrastructure Client could not establish a connection with server "<server>".  Details: You do not have permission to login to the server: <server> .
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1003872  (please see this link from vmware)

What worked
In the vsphere web client in the inventory service the number of vcenter servers, datastores, hosts and all other entities was 0. Thus it pointed out that the vcenter inventory service has a problem. Restarted the vcenter inventory service.
Issue Resolved. now the no errors when trying to login using vsphere client.

Alternatively
If you are still having the same issue then your might be a known issue where it won't login using the domain accounts but logs you in with the local accounts.
Login with the super username admin@system-domain (5.1) or administrator@vsphere.local and on the vcenter server>manage>permissions>add permissions and add your domain group and give administrator rights to it.