Windows can fool some of the pci cards some of the time, but if plug and play is not implemented perfectly (whatever that entails) on the card, then plug and play may not be able to get the interrupt that the device needs.
Solution: turn off (in BIOS) any real hardware com ports that are not needed, and anything else that takes an interrupt, but is not currently in use. It would be unlikely that all your real interrupts are taken, but it could be that there is a 'favoured interrupt' for parallel port cards. IRQ 7 comes to mind. Only one port can have this IRQ at a time. So you need another free IRQ.
Then, also try moving the added parallel port to a different PCI slot, as the interrupt assignments can be affected by which slot you use.