As you may be aware, R 4.2.0 has just been released. For Stan users on Windows this is notable because, along with other new features, the release is accompanied with a new version of Rtools. Rtools is a toolchain bundle used for building packages from source and more importantly for Stan users, used to build Stan models into object files or executables. There are no changes for users on MacOS and Linux.
Whenever a new toolchain is introduced, it is very likely that there will be hiccups and problems. Although the Stan development team has a testing infrastructure to cover some of the possible scenarios, it is unfortunately impossible to cover all of them. We will try our best to update the recommended installation instructions as more information becomes available.
Our recommendation is that, if possible, Stan users on Windows hold off from upgrading to R 4.2. We suggest waiting a bit, so more information on the new toolchain becomes available and our developers gains some more insight to help you with