By default, IM feature is not enabled on Zimbra systems after upgrading. You can use these commands to enable it:
su - zimbra
zmprov -l -v mcf zimbraXMPPEnabled TRUE
zmprov -v mc default zimbraFeatureIMEnabled TRUE
zmprov -v mc default zimbraFeatureInstantNotify TRUE
zmcontrol stop
zmcontrol start
Obviously set the COS to the one you use if it’s not the default.
Good luck! 🙂