This error keeps happening to me - it started in the last couple of days.
I'm using a basic 5pt Round brush (nothing fancy), I highlight an existing element and try to modify it by brushing over the area again (to amend the shape or join the paths). It's never been a problem and usually just redraws to the new shape. Now I get this 'Unknown Error'. I'm not sure if it's a bug and how to report it.