Can you get in as a local administrator? (administrator@vsphere.local)
It almost sounds like the host reverted to having local logins be default, rather than your domain. The "thick" client will push your domain credentials explicitly, while the Web Client (by default) will not - that's why I'm curious if the domain has been removed from default.
Go to Administration --> Single Sign On --> Configuration --> Identity Sources to view/change what the default is :smileyhappy: