I posted the problem to the mailing list and after some advices the problem has been partially solved by changing my pilot device to usb: instead of /dev/ttyUSB1 so I now use libusb instead of the visor plugin to access my palm. You can read the thread here:

http://www.pilot-link.org/pipermail/pilot-link-general/2006-September/002921.html

And here:

http://lists.pilot-link.org/pipermail/pilot-link-devel/2006-September/001604.html

I say it is partially solved because even when I have faster transfer rates and I can sync the basic stuff like address book, calendar and todos I still get errors when I try to do a full sync.