Simon 5 is coming soon!
Simon version 5 is currently in beta testing. It is a massive update, introducing a much-requested feature: syncing the Simon data between multiple Macs, plus a Simon Status service, a Link Checker filter, improved Find filter, iMessage and Speak Error notifiers, a new app icon, and much more.
Important: please note that Simon 5 requires a minimum of macOS 10.12 (Sierra), and will be a paid upgrade from Simon 4 after the beta. Purchasers since September 1, 2020 automatically get a version 5 license (that also works in previous versions).
Re: telnet error
If the Port service doesn't receive what it expects, it will correctly give an error like that. Though I don't know why the actual received value isn't displayed.