since the last update, Rider creates the projects with the wrong structure. It assumes that C# 9 features are enabled and creates the one-liner Program.cs file instead of the usual Program class with the "static void main" function.
This happens with netcore3.1 apps as well as with net5 apps. As far as I know, C# 9 requires net6 though. Maybe that's just a setting somewhere (but I can't find it)?