R: cannot install packages after downgrading from 3.5 to 3.4. Error in dyn.load(file, DLLpath = DLLpath, ...) - r

Question

I had installed R 3.5.1 as detailed in here, but I had some problem with some packages (namely rJava and XML), so I have decided to uninstall R 3.5.1 and downgrade to R 3.4.4 from the Ubuntu 18.04 repositories.
I did that successfully, but now, when I try to reinstall any CRAN package inside R with, for example:
install.packages("ggplot2")
library(ggplot2)
I get the following errors:
Error in dyn.load(file, DLLpath = DLLpath, ...) :
unable to load shared object '/usr/local/lib/R/site-library/Rcpp/libs/Rcpp.so':
/usr/local/lib/R/site-library/Rcpp/libs/Rcpp.so: undefined symbol: DATAPTR
ERROR: lazy loading failed for package ‘ggplot2’
* removing ‘/usr/local/lib/R/site-library/ggplot2’
* restoring previous ‘/usr/local/lib/R/site-library/ggplot2’
The downloaded source packages are in
‘/tmp/RtmpFKBMdd/downloaded_packages’
Warning message:
In install.packages("ggplot2") :
installation of package ‘ggplot2’ had non-zero exit status
>
>
>
>
> library(ggplot2)
Error: package or namespace load failed for ‘ggplot2’ in dyn.load(file, DLLpath = DLLpath, ...):
unable to load shared object '/usr/local/lib/R/site-library/Rcpp/libs/Rcpp.so':
/usr/local/lib/R/site-library/Rcpp/libs/Rcpp.so: undefined symbol: DATAPTR
In addition: Warning message:
package ‘ggplot2’ was built under R version 3.5.1
Any clue how to solve this? Thanks

Answer 1

OK this script to remove all user installed packages works like a charm if anyone is interested. It was taken from here.
# Shamlessly stolen from:
# https://www.r-bloggers.com/how-to-remove-all-user-installed-packages-in-r/
# create a list of all installed packages
ip <- as.data.frame(installed.packages())
head(ip)
# if you use MRO, make sure that no packages in this library will be removed
ip <- subset(ip, !grepl("MRO", ip$LibPath))
# we don't want to remove base or recommended packages either\
ip <- ip[!(ip[,"Priority"] %in% c("base", "recommended")),]
# determine the library where the packages are installed
path.lib <- unique(ip$LibPath)
# create a vector with all the names of the packages you want to remove
pkgs.to.remove <- ip[,1]
head(pkgs.to.remove)
# remove the packages
sapply(pkgs.to.remove, remove.packages, lib = path.lib)