Just a quick note to those who get this error during upgrade:
ERROR: Installation can not proceeed. Please fix your /etc/hosts file
to contain:... (as also mentioned here)
your /etc/hosts should not contain other host-ip addresses than your zimbra server (and localhost)
If other addresses are present, upgrade quits with error above.
As I don't know how to modify the relevant line in util/utilfunc.sh ( a perl expression), I simply commented out the other servers in the hosts file before running the upgrade.
Which ran fine. Afterwards I removed the comments.