When opengl window is on 3d mouse support is temporally disable.
Should be normal if opengl window is the current active window but same scenario if Houdini window is active. Kinda weird.
When opengl window is closed everything go back to normal.
I guess this is hard to investigate if you don't have any 3d mouse but maybe you have an idea.
http://www.sidefx.com/docs/houdini/basics/3dmouse.html