It doesn't make sense that LPD did not work while IPP did, as the UFR2 driver does not support the IPP protocol so it creates a queue using the LPD protocol. If you look at the queue that gets created (by looking at Options & Supplies > General > URL) you will see that LPD is used in the path regardless of what protocol you select when you create the queue, as shown below.
cntdbmu://MCD
UFR2_LPR/iRC2550UFR2/lpd://172.17.12.150/
Also, if it was working on 10.6.3 it should have kept working on 10.6.4. I have not found any issue with the UFR2 v2.10 after updating the 10.6.3 Mac's to 10.6.4. Maybe in your case there was some other cause for the driver not working and the selection of another protocol tricked the Mac into thinking it was a new queue?
