Can not open existing drawings‚ with inCAD

Problem:

I can not open any existing drawings‚ with inCAD.‚  The project manager appears, but when i select the drawing to open only see a message "nil" at the command line.‚  I can, however,‚ create new drawings and all of the inCAD commands work properly.‚  If I start AutoCAD without inCAD then I‚ am able to‚ open up the drawings, but then of course the inCAD commands are not available.

Command: assistant
nil

Solution:

Go to Tools > Options, and on the System tab‚ ensure that the "Single-drawing compatibility mode" option is unchecked.