hm, sounds like yet another plymouth problem; not a kubuntu problem. Why canonical chose to put it in an LTS release I'll never know.... Anyway, my recommendation would be to boot from your ubuntu disc and edit your grub list to disable quiet boot. You won't have a bootsplash anymore; just lines of console output, but it'll at least be working. You'll have to run this command (terminal)
then change this line by deleting the words quiet splash, leaving everything else (including quotes)
gksu gedit /etc/default/grub
that should do the trick.