Yes, reverting back to rstan 2.19.3 will fix the problem. As of now, rstan 2.21.1 is only available as source on CRAN so when you just uninstall rstan and install it again but then refuse to compile from source, it will give you rstan 2.19.3 directly. In a few days rstan 2.21.1 will also be available as compiled package in which case the approach above does not work anymore and you have to go for:
packageurl <- "http://cran.r-project.org/src/contrib/Archive/rstan/rstan_2.19.3.tar.gz"
install.packages(packageurl, repos = NULL, type = "source")