Sounds like your desktop datebook.dat file is corrupted.
If you are not using at least version 4 of the Palm Desktop Software, then upgrade. Version 4 automatically fixes this datebook corruption when it reads in the datebook.dat file.
If, for some reason, you are unable to upgrade to version 4, you can try using the datebook repair tool created by Tobias Oetiker http://oetiker.ch/dtbrc.cgi. It may solve your corruption problem, which would then permit you to hotsync ok. If it does work, you'll need to Hotsync with desktop overwrites handheld. Oh, and be sure to give appropriate thanks to Tobias for the fine tool.
If the above fails, you may need to consider abandoning the desktop version of the corrupt datebook file. Obtain dbscan, install it on your Palm, then run it to ensure that the datebook on your Palm is corruption free. Then perform a hotsync back with the datebook conduit set to handheld overwrites desktop. This should put the clean copy from your Palm back on the desktop.