Are you using the original UBCD 5.1.1? Yes, from site
You wouldn't guess how many people confuse http://www.ultimatebootcd.com/download.html
with something else or download some _other_ "UBCD" from other places.
Have you tried downloading it anew from the original site and checking the md5 checksum? Not yet, but I am sure MD5 is original
Then please "be really sure"; see http://wiki.ultimatebootcd.com/index.php?title=Tutorials:Calculating_Hash_Sum
Are you booting from CD, or from USB drive? USB
How did you install it on the UFD? Did you use ubcd2usb? Other USB-related method? Multiboot with other tools?
Could you please provide specific info about the system where you are booting FD UBCD (the FreeDOS entry in the original UBCD 5.1.1)? What?
All info you could provide. Mainboard, BIOS version / (U)EFI, specific settings that may change its boot behavior, boot order, boot media, extra bootable cards...
Have you tried booting FDUBCD on another system so to compare the results? Not yet
Well, it would be helpful to know what happens with that specific UFD when booting other systems. And, the other way around, booting other FreeDOS versions in the same system where fdubcd showed a problem. All feedback and comparisons can be useful so to solve this.
How did you trace it specifically to himem?
Have you tried to boot with ubcd52a1 or ubcd52a2 (they are not really ready for normal usage, but they may give you a different behavior during the boot of fdubcd)?