I have exactly the same problem than @mafosein this post. As the threat is already closed, but not answer posted, I start a new post.
Since a few days I get the error "Error listing packages" when I start RStudio and in the console the notification "Error in readRDS(pfile): cannot read workspace version 3 written by R 3.6.0; need R 3.5.0 or newer"
RStudio is running in windows on a citirx workspace.
The default serialization format for R changed in May 2018, such that new default format (version 3) for workspaces saved can no longer be read by versions of R older than 3.5 (you can see relevant commits in the CRAN mirror below)
R 3.5 was a major update that required new installations of all packages (see post below)
All of which is to say that I would recommend updating your version of R.