@andrjohns @Charles_Driver @JacobCoblentz
In a clean R session without any old stanfit objects being autoloaded, try doing
install.packages(
"https://win-builder.r-project.org/8bPDZPDG0522/rstan_2.21.2.zip",
repos = NULL, type = "win.binary")
then
example(stan_model, package = "rstan", run.dontrun = TRUE)
plus whatever examples were not working before.