This happened today. The already running application didn't started today.
I pressed the "Restart" to try to "wake it up" and it throwed that message in a pop up.
But then, out of the nothing, it started working.
In the last months, every now and then, it happens that the instance doesn't start.
But today it was the first time that Restarting the instance didn't worked.
It usually happens with production apps, as the dev ones works.
Obviously this is a problem in the shiny server. But why?