It’s not SSL. I checked this version. I turned https access off on management interface – via http a got same error.

Yes, you can upgrade (I did it from command line).

Quick upgrade procedure (including command line via SSH):   https://www.cisco.com/c/en/us/support/docs/security/email-security-appliance/118547-technote-esa-00.html

We also have EOL old product (but it' works fine!), and have some problem :(   Product: Cisco IronPort C150 Messaging Gateway(tm) ApplianceModel: C150Version: 7.5.2-014Build Date: 2012-03-06Install Date: 2012-03-06 23:35:48   I think, cause maybe in ...