Can you check the output of these commands to see if convertd is enabled
Code:
su - zimbra
zmprov gs `zmhostname` | grep zimbraServiceInstalled
zmprov gs `zmhostname` | grep zimbraServiceEnabled
If convertd is not listed in the output of the enabled services, Can you enable it, re-index the account and let us know how it goes.
Code:
su - zimbra
zmprov ms `zmhostname` -- +zimbraServiceEnabled convertd
zmconvertctl start