You really shouldn't start modifying the MySQL tables unless you know what effect your changes will have.
Is this backup failing on a particular account? Do you know which account is having the problem? I'd suggest you export the data from the account (if you can't perform a backup) then import the data into a new account. When you've done that and made sure the account is OK you can remove the tables for that account. Have a look at this
wiki article to find out which tables you need to remove. Do this after you've shutdown Zimbra and made a backup of your /opt/zimbra directory and with no mail flowing into the system.
If you get that mailbox back to a working state I'd suggest you install Zimbra on a supported platform so you can take advantage of contacting support to help you fix problems.