mv ~/.Rprofile{,.bak}
 mv .RData{,bak}



I think the issue is a nonstandard `options('prompt')`. I (re)moved `~/.Rpofile` and the `.RData` file for the working directory of the session and no longer had the blocking/endless waiting/frozen issue. 

prompt as an issue also showed up when octave's default prompt changed:
https://stackoverflow.com/questions/27742075/emacs-freezes-when-running-an-octave-code-block-within-org-mode


setting my prompt is now pretty complicated



 .First <- function(...){
 updatePrompt <- function(...) {
 # Emacs Speaks Stats, use default prompt
 if(options('STERM')=='iESS') 
 options(prompt="> "); 
 # Rstudio -- time but no colors
 else if( any(grepl("RStudio", .libPaths())) )
 options(prompt=format(Sys.time(), "\n# %X\n#> "))
 # command line R
 else 
 options(prompt=format(Sys.time(),
 "\n\e[38;5;197m# \e[38;5;27m%X\e[0;0m\n "))
 return(TRUE)
 }

 # add prompt if we're in interactive mode
 if(interactive()) { 
 addTaskCallback(updatePrompt)
 updatePrompt()
 }
 }


https://github.com/LabNeuroCogDevel/dotfiles/blob/master/Rprofile/.Rprofile