From Zimbra :: Wiki

This tends to happen because Zimbra can't read the key or the certificate of the LDAP/IMAP proxy service on startup. This issue is specific to commercial certificates.

Here is what you can do:

1. You want to check to make sure the private key is not encrypted.

2. Manually link the cert to the services OR simply redeploy the certificate to relink the cert to the services.

(a)cp /opt/zimbra/ssl/zimbra/commercial/commercial.key /opt/zimbra/conf/slapd.key
(b)cp /opt/zimbra/ssl/zimbra/commercial/commercial.crt /opt/zimbra/conf/slapd.crt
(c)cp /opt/zimbra/ssl/zimbra/commercial/commercial.key /opt/zimbra/conf/nginx.key
(d)cp /opt/zimbra/ssl/zimbra/commercial/commercial.crt /opt/zimbra/conf/nginx.crt

3. Restart the Zimbra services.