rstudio-server "unauthorized user" error on google vm

I have a Google VM with Rstudio server installed. Everything was working fine and I was able to get in with my username and password. I haven't used this VM for a couple months and now I tried again. I am getting the Rstudio login page when typing <server-ip-address>:8787, but when I enter my username and password I get the "unauthorized user" error. I'm not sure what changed.

The only similar post I found was this Cannot log in to AWS RStudio Server and Workbench, instead see 'Error: Unauthorized user.', which turned out to be an AWS problem.

I can SSH into the server with my user. It is not a password problem (since then you get a different error). Any suggestions?

This is how I solved it:

First, I checked the rstudio-server log file:

sudo cat /var/log/rstudio/rstudio-server/rserver.log

where I found the following message:

WARNING User user_name could not be authenticated because they do not belong to one of the required groups (analysts,admins,rstudio-users); LOGGED FROM: bool rstudio::server::auth::validateUser(const string&, const string&, unsigned int, bool) src/cpp/server/auth/ServerValidateUser.cpp:125

When trying to add my user to one of the groups analysts,admins,rstudio-users, I found that none of them exist in my system. So first I added a new group called rstudio-users:

sudo groupadd rstudio-users

Then I added my user to the group:

sudo usermod -a -G rstudio-users user_name

And then I restarted rstudio-server:

sudo systemctl restart rstudio-server

And that solved it.