Over the weekend I've been renmaing my Domain Controllers to fit in with the other Servers (dc1 -> dc01). The Next day, I couldn't log into vCenter anymore with my Domain Account, neither with Windows Session Credentials nor Direct Input. I got this very cryptic error "N3Sso5Fault13InternalFault9ExceptionE":
Took me a bit tinkering, but then I rembemered I renamed my DC's and hadn't updated them in vCenter. So I logged in with the vCenter SSO Administrator Account, readded the Authentication Source and all was well, even the Windows Session credentials worked again!