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: Failures that won't recover
What is the exact error in the Failures page of the Monitor window?
Do you have Smart Change Detection switched on, with text specified for the Start or End blocks? If it can't find that text, it'd give an error saying so (which is a useful technique to ensure that the page includes essential output).
Perhaps attach screenshots of the page errors, if that helps clarify.