the credentials supplied conflict with an exsisting set of credentials"
I have gotten this error in the following situations:
I was logged into the computer as a local user. I tried to access a share that was admins only. It prompted for domain credentials, which I provided, but that error popped up.
Once I logged in as a local admin, it still prompted for credentials, but I no longer got that conflict.
I got the same result when trying to join a computer to the domain.
I have no idea what caused it, I just know what fixed it. =)