include_app causes Error in enc2utf8(x)

I tried to write an interaktive book using bookdown and including shiny apps in it. However calling

knitr::include_app('https://yihui.shinyapps.io/miniUI/', height = '600px')

gives me a
Error in enc2utf8(x) : argument is not a character vector
I am running RStudio on a Mac. If you have any ideas I would be more than grateful.

Best Wishes
Philipp