On ubuntu 14.04 LTS, sometimes my computer fails to launch Nautilus. This always happens when I have been working on it for a while (maybe after suspending and waking it again?), never when I just turned it on.
I suspect this is related to gvfs filesystems mounted regularly and then left alone for a while.
and similar commands give
Could not register the application: Timeout was reached
sudo nautilus does launch it.
This sounds related, but I don't understand the solution.
Logging out and back in solves the problem.
I know that other people have had the same problem in the past. I do think this is a serious issue. I reinstalled ubuntu recently (for other reasons) and the problem is still there. Might be worth mentioning that I have nautilus-dropbox installed.