It is very strange that you can't boot the CD on the 2 servers, but that it works on other PCs. It is also strange that the windows CD will boot on the server PCs. The windows setup CD and UBCD use both the no-emulation option to boot from a cd. The windows CD can be booted by the bios (supports booting CDs in no- emulation mode) so isolinux on the UBCD should load. Don't you see anything when you try to boot from UBCD on the servers (something like:
ISOLINUX V3.17 <date> Copyright (C) 1994-2008 H. Peter Anvin)?
You can build the iso again but this time you use
isolinux-debug.bin (rename to
isolinux.bin) of the syslinux archive (/core/isolinux-debug.bin. Burn it to an CD-RW if you have one (to save a CD-R

). If the boot sector of the cd gets loaded, you should see some extra output. Write it down exactly. You probably have to post the output to the syslinux mailing list, so they can solve it. Provide enough info about your hardware.
You also can try to boot UBCD from an usb stick (use the ubcd2usb script). This probably also won't work because most PCs can boot CDs which have isolinux, but not always from an usb stick due buggy bios usb code.
After a quick look at the internet, I found websites which describe booting CDs with isolinux on the hardware that you have, without any problems (not completely true, but the isolinux part of the cd worked at least).
You also can post to the syslinux mailing list if we can't solve the problem. Describe your hardware in great detail, etc.
I don't know if it can help or not, but you can always try: remove every unnessecary device / card temporary out of the server that you don't need directly, so that you only have the cdrom drive atached (remove the hard disks connections temporally too). Remove also some RAM if possible (normally shouldn't matter, but you never know). Try if you can boot the server now with UBCD.
You also can download another live cd (e.g damn small linux, puppy linux, parted magic) and burn it. They all use isolinux as bootloader. See if you can boot them.
You also can try to make ubcd with grub4dos as bootlaoder instead of isolinux:
See the post of ilko at:
http://www.boot-land.net/forums/?showtopic=4486&st=10
Problematic CD-ROM controllers
At least one Promise ATA-type controller with BIOS (model number unknown) does not work with ISOLINUX since it hard-codes the location of the El Torito boot catalogue from the Windows 2000 CD-ROM and will only boot from this location!!!!!! The reporter of this bug was able to get it to boot by hacking mkisofs to put the boot catalogue in the same location as the Windows 2000 one.