I ended up losing a few hours of work today to this issue (trying to knit an Rmd file I created on OS X on a Windows 10 laptop, running R 3.5.0). At first, I was getting "yaml could not be parsed" errors, which I thought were due to some character encoding issues.
It was only after coming across this thread did I try restarting RStudio without loading an Rmd file -- and I still got yaml errors. Weird.
I updated to the latest daily release and the issue was resolved.
One oddity I have noticed is even after running install.packages("yaml", dep=T) and the package getting reinstalled, it does not appear in my packages list in RStudio. Is this normal?