I believe RStudio Server requires users to have their own directory in /home/. Upon initial login, RStudio Server generates the R/ folder in that path to store installed libraries. It may also create other necessary files to run.
I think what you mean by the default user is the default runAs user of rstudio-connect. If you check the /etc/passwd file for this user, I don’t think it’s a loginable user. If you can not login, you don’t have the /home/ path and the necessary files aren’t created. To login as rstudio-connect, you would have to modify passwd to allow logins.