OK. I got the problem fixed. The suggestion from cderv above is corrrect but a bit incomplete. Simply reinstalling the tinytex package with install.packages() doesn't fix the problem. Evidently what is required is to reinstall the entire TexLive that is called by tinytex. That is done by entering in your R console the following command:
tinytex::reinstall_tinytex()
This evidently reinstalls TexLive (not just the tinytex package). It will take a few moments to complete. Then shut down R and reopen it and repeat knitting your Rmd file. This may take quite a few minutes to complete, as TexLive may have to reinstall a whole series of Latex packages (not R packages). But it eventually completed and produced a correct pdf file for me. Once this first call to tinytex completed, all the subsequent calls went lickety-split as usual.
Larry Hunsicker