file sharing problems

I have two machines, dual G5 powermac and G4 powerbook. Both are running 10.4.7; configured identically as far as I can determine (personal file shareing, firewall, etc.).

File sharing between the two worked fine until recently.

The powermac can still connect to the powerbook as expected. Go to "Network" in the finder, the powerbook is there. Double-click the powerbook's icon, dialog pops up with "Connect to the file server ..." as expected; with radio buttons for 'guest' or 'registered user'. After entering name and password, list of available shares appears. Select one and the network disk is mounted as expected.

However, the reverse situation doesn't work. On the powerbook, if I navigate to "Network" in the finder and double-click the powermac's icon, I get a dialog box named "FTP File System Authentication" - no radio buttons for guest vs. registered user. Filling in name and password eventually gets an error dialog with the message "the alias {powermac} could not be opened, because the original item could not be found".

From finder or a terminal window, the powermac's alias in the powerbook Network directory cannot be changed or deleted.

I don't even know where to start with this; no idea on whether the problem is on the server side (powermac) or client side (powerbook).

Any ideas how to fix this?