A bug was opened for this:
http://bugzilla.zimbra.com/show_bug.cgi?id=15241
The work-around until the bug is fixed is to delete the appointment that is causing the exception.
zmmailbox -z -m username@domain
di <item number>
exit
<item number> can be found in the sync log when the exception is logged.
2007-03-08 10:41:36,689 WARN [http-443-Processor99] [mid=321;
name=kunkled;DeviceId=3BC9A9F061A7399F5DD8 D7EC435FABFE;DeviceType=PocketPC;SyncCmd=Sync;clas s=Calendar;folder=10;
item=44055;] sync - Unexpected exception encoding server change
java.lang.IllegalArgumentException: DayOfWeek field must be in [0, 6] range
I want to thank Zimbra's support team for their quick response to my support request. They had a developer working on it with in a couple hours after I logged the request.
Jason