This is by design -- fix for bug 32969, implemented in 5.0.12. Certain addins (especially iTunes) caused problems for our sync, where the sync thread would just die. So we decided to tell the user this, and disable the addin. If we find more addins that cause the problem, we will add them to the list. The list of addins is controlled by a registry key:
HKCU\Software\ZIMBRA\AddinList
HKLM\SOFTWARE\Zimbra\AddinList |