Recently we have had a few support cases where a customer was unable to log in to the firewall via WebUI after the firmware update. But SSH access worked fine.
It turned out that during the update process the server certificate used for the WebUI is lost.
Config with v6.0.4 (it does not happen with „self-signed“ only):
config system global„What’s new with FortiOS 6.2: Update issue with certificate for WebUI“ weiterlesen
set admin-server-cert "self-signed"