Current issues in ZCS 8.6
Current (Known) Issues in ZCS 8.6 Build 1153
Bug #97304
Upgrading from 8.5.1 to 8.6 doesn't check the local proxy variables and it fails the upgrade. When upgrading, the Script validates the license against https://license.zimbra.com. If your Server is behind a proxy the upgrade script (and also the license activation itself) ignores the Proxy System Variable.