Did Saruman Believe He Could Master The One Ring?

[the phantom]

I think that the error in question was Sauron believing that the hobbit and the palantir were in Saruman's control.

Ah, but SPM's point was that the error Gandalf spoke of would take some time to clear up, where as the error you are speaking of (believing the palantir and the hobbit were at Isengard) is a quickly corrected error. That makes me think that the error that would take some time to clear up is Sauron's mistaken belief that the Ring itself was at Isengard, had been at Isengard, or was at least with those who had been at Isengard.

EDIT: Btw, I just thought I'd mention that every bit of speculation we do based on Gandalf's words is guess work at best. Gandalf did not know everything. He has been very wrong before, and has a habit of stating things as fact that were not truly known.