As we had an issue with Java going bad after an upgrade, it turned out the upgraded release was so old that some of the components had been gone and some new arrived.
Java components are in lib folders, and Java loads the folder, not the component, thus old components were loading as well.
So by chance, we browsed into lib, zimlet, bin folders and found modules that aged back to the installation of the previous release.
Backing those up in a safe location, then deleting them from the folders and restarting zimbra solved the issue.
Well, I should also mention that the upgrade was from 5.0.11 NE to 7.1.4 OpenSource, so some modules remained belonged to the NE, but I think not all...
Java going bad before the update was most certainly due to corrupt mailbox database.