This is basically a known problem. The EISA on-board SCSI controller in the HP Netserver machines occupies EISA slot number 11, so all the ``true'' EISA slots are in front of it. Alas, the address space for EISA slots >= 10 collides with the address space assigned to PCI, and FreeBSD's auto-configuration currently cannot handle this situation very well.
So now, the best you can do is to pretend there is no address range clash :), by bumping the kernel option EISA_SLOTS to a value of 12. Configure and compile a kernel, as described in the Handbook entry on configuring the kernel.
Of course, this does present you with a chicken-and-egg problem when installing on such a machine. In order to work around this problem, a special hack is available inside UserConfig. Do not use the ``visual'' interface, but the plain command-line interface there. Simply type
eisa 12 quit
at the prompt, and install your system as usual. While it is recommended you compile and install a custom kernel anyway.
Hopefully, future versions will have a proper fix for this problem.
Note: You cannot use a dangerously dedicated disk with an HP Netserver. See this note for more info.