I did get the default.rule error (starting from terminal) when I had installed both the nautilus_3.30.0-0ubuntu1~cosmic1 and the tracker_2.1.3-1~cosmic1 packeges.
As I mentioned, at that time nautilus did not start, the small wheel on the screen just kept on turning.
Then, after downgrading tracker_2.1.3-1~cosmic1 back to the official tracker_2.0.3-3, I did get nautilus_3.30.0-0ubuntu1~cosmic1 running.
However, the thumbnailer did not work then.
Sorry, I am not familiar with the backtrace system.
I did get the default.rule error (starting from terminal) when I had installed both the nautilus_ 3.30.0- 0ubuntu1~ cosmic1 and the tracker_ 2.1.3-1~ cosmic1 packeges.
As I mentioned, at that time nautilus did not start, the small wheel on the screen just kept on turning.
Then, after downgrading tracker_ 2.1.3-1~ cosmic1 back to the official tracker_2.0.3-3, I did get nautilus_ 3.30.0- 0ubuntu1~ cosmic1 running.
However, the thumbnailer did not work then.
Sorry, I am not familiar with the backtrace system.