Application Event log is full of these, letsencrypt_win_simple
Log Name: Application
Date: 7/1/2021 958 AM
Event ID: 36428
Task Category: None
UriFormatException: "Invalid URI: The URI scheme is not valid."
I'm guessing it doesn't like one of the lets encrypt URIs, but I have no idea which one anyway to get additional logging detail from somewhere to find out?
The best method would be to run letsencrypt_win_simple manually and check the renewals. You can normally find the exe in C:\SolidCP\Server asp.net v4.5\bin\LetsEncrypt.