Please try to delete the Touch action for the Blend tool. Maybe it is being activated when you use your pen.
To do this, please go to Rebelle's menu: Edit > Keyboard Preferences > Tools (hold) just like you see on the picture:
Then click the "Blend" tool and set it to "None" in the bottom. Confirm with the OK button in the top right corner and try again.
Does the pen work now?