Can't knit since update to MacOS Catalina (10.15)

After updating to MacOS Catalina, RStudio no longer works, so I upgraded the latter to v 1.2.5019.

Now, when I try to knit an Rmd file, I'm told: "Rendering R Markdown documents requires an updated version of the digest package. Do you want to install this package now?"

When I select Yes, I get a window appearing too briefly for me to read. Something appears to be installing, but when I try once again to Knit, the whole cycle repeats.
Installing "digest" manually results in the same loop.

Any suggestions? Thanks in advance.

Are you able to install digest in a clean R session? E.g. from the terminal:

R --vanilla --slave -e "install.packages('digest')"

I have stumbled upon a solution that has worked so far. If I double click on an Rmd file that has not been opened since migration to Catalina, I am unable to knit the file, and I encounter the error described. However, if I instead open the Rmd file from within RStudio using the menu, then knitting works fine and the Rmd file also subsequently behaves as expected. So it appears that for each Rmd file, I will need to open it the first time from within RStudio and then can use it as before.