Problem solved - one of the mates in the assembly had lost its reference to a face. The study appears to be running satisfactorily now. Strange error message!
it is strange
it sounds like you're saying it threw that error because there was no path to a fixture for the one part
i would have expected something like singular matrix or some other instability error
the analysis should run basically instantly the way you describe, if it takes longer, there is probably another problem