iSyncing iCal and address book with Sony Clie (Palm OS)

This was a hard one. Earlier this month I managed to successfully sync iCal and the address book with my Sony Clie. However, I recognized recently that new entries in the calendar or the address book have not be synced any more. The following error occurred in the HotSync.log:

iSync Conduit starting 5/25/08 10:15:08 PM

iSync Conduit: received NULL message, disconnecting

Some googling revealed a rather simple solution: the use of groups in the address book prevents isync from a correct sync. Just remove the groups (contacts will remain) and the sync is working again. However, I liked to organize my contacts that way and now you a bunch of contacts just in one big list. Additionally, I would like to use the Palm concept of categories (puts a label to each contact) within iCal, but I did not find a solution to that problem. I do not know how to access the category information from the Mac address book :-(.