Error in compileCode()

I am able to download rstan now via the link. Unfortunately, I get the following error message now;

Error in stan_model(file, model_name = model_name, model_code = model_code,  : 
  Boost not found; call install.packages('BH')

To state the obvious, BH is installed. I did notice one other earlier message mentioning the same issue;

However, the solution in that post was to simply restart R. That doesn’t work for me…
Sorry for the many questions. I have been using Stan for years now, and only recently has it started to give me issues that I have never had before.
Alex