Bluetooth: Only One-way File Exchange


I just got a new PowerBook, running Mac OS X 10.4.2. At home I have an iMac running OS X 10.3.9. Both are Bluetooth enabled. I can send files to my PowerBook from my iMac, but I can't go in the other direction: it says 'unable to validate device.' In both cases, the devices are on the 'favorites' list, and it looks like the settings are OK. Any idea what's going on?
I deleted the pairing on both computers. I tried to pair again using the iMac, and it paired fine with the PowerBook. I then deleted the pairing again, and tried from the other direction. When I tried to pair it from the PowerBook side, it showed the iMac, but wouldn't pair with it. Any ideas?