the solution is to create a derivative class for fix rigid,
that knows how to handle rigid bodies with degenerate
moments of inertia. those are used to define the orientation
of your rigid object, so that it can be moved as a whole
(by computing the movement of the center of mass and the
rotation around it seperately). that would need to be somewhat
adjusted to your specific problem, but not really a big deal.
just have a look into a textbook on the mechanics of rigid bodies
in case you have additional questions.
This was an issue with too tight a tolerance on computing the
moments of inertia for rigid bodies with lots of atoms. I posted
a patch.
Note however that these commands
fix e4 tip rigid single
fix freezeXZ tip setforce 0.0 NULL 0.0
indicate you really don't need a rigid body. If you just want
the group of atoms to move collectively in y (not in x,z) then
you can use fix aveforce and no rigid body.