For others who face the same issue (or future me), the solution @rramiro suggested didn't work for me on Rstudio Server version 1.3.1093, but the general concept worked. Based on the link he/she kindly shared:
user files are stored in three separate locations
~/.config/rstudio/ All user settings (Global Options)
~/.local/share/rstudio/ All open files and tabs
~/.rstudio/ All active and suspended sessions
On my computer, the ~/.rstudio had only one empty folder inside, and renaming it and restarting the rstusio daemon via systemctl restart rstudio-server.service didn't help. What worked for me was renaming the ~/.local/share/rstudio/ via:
mv ~/.local/share/rstudio ~/.local/share/rstudio.old
and the restarting the daemon:
systemctl restart rstudio-server.service
and then refreshing the browser. Just as a headsup, the rstudio took about 20 seconds to load after this (probably because it had to create all those file we practically removed).
Nevertheless, special thanks to @rramiro 