I just updated to R 4.0.0 and Rstudio 1.3.959, and also updated Rtools and rstan at the same time, and now am having the same issue reported here (cannot view progress on viewer). The model fitting is successful, and once it is done I can view the results in viewer, however when I try and refresh the Viewer window to see progress while it is running I get the “The process cannot access the file because it is being used by another process.” error message