There's a new build of ArtRage 6.0.8 available for download.
The following bugs and issues have been addressed.

Crash on undo/redo of layer parameter changes (opacity, blend mode, etc)
Frisket (stencil) widgets in guide/ruler mode doing single-dab and cant continue drawing
Loaded paintings can prevent 'undo' working.
Pasting layer from clipboard could corrupt layer ID.
Retina OSX - rotation and scale of reference images improved.

Tightened the 'About Box' retaining screw which prevents it falling down the canvas.
Okay, that last one isn't true. The 'About Box' retaining screws never come loose.