Thank you!!!! So so much! I downloaded version 4.0.5 (I had 4.1.0 before) and by reverting to the older version, it worked perfectly.
So it looks like shiny apps don't launch in RStudio with R version 4.1.0. I'm on windows, maybe it works on other OS?!
Anyway, thanks so much, really appreciate your help.
Generally, is there a way to let the people who make Shiny know about this? I'm thinking it would be helpful to highlight it and save others the stress.