Serial port '/dev/ttyACM0' not found
On Linux, did you happen to select 14400 or 28800 baud at any time? Even once, even for just a split second?
Linux doesn't support these speeds and the RXTX library doesn't support Linux not supporting them, so the result is RXTX loses all ability to talk to your port, as if it no longer exists. If you quit and restart Arduino, you'll get a message about stale lockfiles, but then as soon as you try the serial monitor again, it remember the bad baud rate and the problem happens all over again. When I hit this, I had to edit the prefs file in the .arduino dir to "fix" it (back to a working baud rate).
Then again, you could have a completely unrelated problem. Just beware of those booby-trapped baud rates in Linux. They'll ruin your whole day.