With the release of Mac OS X 10.4 more than a year ago, iSync no longer has anything to do with the synchronization of contacts, calendars and tasks with .Mac - that is the responsibility of .Mac Sync. You should probably post you query in the .Mac Sync forum, here:
http://discussions.apple.com/forum.jspa?forumID=957
You may be able to deal with the error message by resetting the Sync Services framework—its truth database, actually—by launching iSync, opening the Preferences… dialog, and pressing the Reset Sync History button. That will leave your data intact and your sync clients registered, but reset the truth database and framework to behave in such a way that your next synchronization event will appear to be the first event you have ever performed.
The 'incomprehensible archive' the error message refers to is likely on your Macintosh.
Before proceeding further, back up your contact and calendar files using the built in provision to do so in both the Address Book and iCal. If resetting the database does not correct your issue, you can launch System Preferences, open the .Mac pane and press the Advanced tab. In the lower left corner is a Reset Sync History button. Press that, and in the dialog which appears, configure .Mac to overwrite the data files for one OS X application on your machine at a time, starting with contacts. Make sure that you overwrite your computer data [which you have just backed up, and can therefore restore if that is necessary] with the data from .Mac.