It sounds like there might be a couple different things going on here:
First, There is a known issue that causes this error message when you leave a tab running RStudio.cloud but do not interact with it for some period of time. We haven't exactly identified how long it takes for this to occur, but some of our testing showed it could happen after as little as 15 minutes of inactivity. We're looking at possible fixes for this issue, but refreshing the browser should be enough to fix it in the meantime.
Secondly, it's possible some other types of problems could cause this error message. For example running R code that exhaust the memory on the system (causing the IDE to crash). Do you happen to know if the student was running code that could cause this type of error?