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Article Information
This article applies to the following ZCS versions.
ZCS 4.5

Note: This issue occurs mostly on Mac servers.

Before doing anything else, try Resetting Your Mobile Device.

If a resync doesn't fix your issue, and you're seeing this in /opt/zimbra/log/sync.log:

[mid=3;name=laan.vd.r;DeviceId=A8C56474222A0CACB274C839DEC2C55A;DeviceType=PocketPC;SyncCmd=Sync;class=Calendar;folder=10;] sync - Unexpected exception
java.lang.NoClassDefFoundError
at javax.swing.SwingUtilities.isEventDispatchThread(SwingUtilities.java:1270)

You've hit [bug 13974]

The fix is straightforward:

su - zimbra
tomcat stop
su - root
mv /opt/zimbra/libexec/zmtomcatmgr /opt/zimbra/libexec/zmtomcatmgr.fcs
ln -s /opt/zimbra/libexec/zmtomcatmgr.unrestricted /opt/zimbra/libexec/zmtomcatmgr
su - zimbra
zmlocalconfig -e tomcat_java_options="-client -XX:NewRatio=2 -Djava.awt.headless=true"
tomcat start