This week we updated a VCSA 5.5u1 from a new customer to 6.5->6.7 and this customer only use "root" to login into WebClient or better Windows vSphere Client in all the years. I also think that in earlier version it might be a default.

But.. after migrating from 5.5 to 6.5 this was not a valid user within vCenter anymore and was unable to login. We grand permission to localos\root by using administrator@vsphere.local and then it works as before.

Regards,

Joerg