RStudio server: R is taking longer to start than usual

I have an RStudio server (open source, version v1.2.5033) running on Arch Linux.
Although it is working fine, one user has just encountered a problem.
When they follow the URL, they can log in using the normal log in dialog box, but the RStudio interface itself does not come up on the webpage. Instead, they get this, though none of the buttons do anything.
image

I killed the R session process in that user's name, in the hope that a new one would be spawned. But nothing seemed to change. I could restart the Rstudio server, but that would presumably destroy everything for the other users who are not experiencing trouble.

Is there anyway that I can restart a RStudio server session for just one user?