I take it this is a wired sync using desktop manager (v4.2?)?
Whats the exact error it throws up and at what point in the sync process?
Try configuring DTM to sync only one application e.g. Address book at a time. this might help you narrow down what is upsetting the sync. e.g if it syncs happily with all applications but address book you know it's probably a contact that is the problem.
you might find that enabling advanced logging in intellisync will help you - but only if you fancy trawling through the log files it produces...:
http://www.blackberry.com/btsc/search.do?cmd=displayKC&docType=kc&externalId=KB01451&sliceId=SAL_Public&dialogID=12248145&stateId=1%200%2012242697
