Error: Rstudio unable to establish connection with R session

When I start my Rstudio, no matter which button I click or whatever I do, this "Error: Unable to establish connection with R session" pop up.
I have no Antivirus.
I have already tried to uninstall and reinstall R and Rsdtudio, also the previous version.
clear all old files corresponding to R/Rstudio.

But no result, there is the same error,
what should I do?