Upon burning livecd-71145-dbg.iso to a DVD+RW disc and hooking up my Dell Dimension 4100 via serial-usb cable to PuTTY.
The culprit seems to be related to the keyboard I'm using, a Microsoft Wired Keyboard 600. With that not plugged or loose debug tends to load with my USB mouse, running normal mode more often than not I have to plug out my keyboard to get the mouse to work. There is only two USB ports to use.
As the keyboard stopped working outside the boot menu I could not press "bt" and attempting to make an ISO with a modified freeldr.ini to use /KDSERIAL resulted in "Assertion '*REDZONE_HI(Block) == REDZONE_MARK' failed at /srv/buildbot/Build_GCCLin_x86/build/boot/freeldr/freeldr/lib/mm/heap.c line 173", so I can't give you a backtrace I'm afraid.