question

It's possible there's some node that's corrupted and can't be saved. You could try running this script to see if any of the nodes in the tree can't be saved. From the Debug menu at the top of the screen open the Output Console and then drag it so that it undocks. Now open the Debug console. Paste this code into the debug console and then push the green run button.

forobjecttreeunder(model()) {
	treenode obj = a;
	print(obj);
	savebyname(obj, "test.t");
}

If you get an error at one of the nodes you might be able to look at the Output console and see what node it was just trying to save.