What version of R are you running?
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.