My assumption is that OS X Server 1.2 does not support OS 9.1 in its "Blue Box" environment. Since the OS X Public Beta Classic environment does not support 9.1, why would the even older (and barely supported) OS X Server?
In fact, the last I had heard was that OS X Server didn't support OS 9 at all in the Blue Box. I could be wrong, but I recall 8.6 being the last supported OS for the Blue Box environment.
Now don't confuse the OS X Server Blue Box with the NetBoot Disk Image. I am running 9.04 in the NetBoot environment, and may experiment with 9.1. But it is a totally different animal than the Blue Box.
BTW, anyone running 9.1 on NetBoot yet? How is it?