Your boundingObjects appear to be almost entirely boxes, is there a reason why you’re not using the Webots Box primitive? You might be adding unnecessary complexity using a mesh in this case. I quickly tried to replace those on the feet with normal Boxes and it seems to have an effect, I didn’t check further than that but might be worth a try.
Also, keep in mind that by not specifying the physics node at the Robot level, the default parameters are being applied (for instance, a mass of 1 is added there).
I wouldn’t worry about the position warning yet since as you mentioned you’re not actually controlling the robot, it could be violating the limits when it bounces. Just make sure the limits on your motors/hinges are set properly as described here. The relevant part is:
Finally, note that when both soft (
maxPosition (motor limits)) and hard limits (
maxStop, see JointParameters) are activated, the range of the soft limits must be included in the range of the hard limits, such that
minStop <= minPosition and
maxStop >= maxPosition. Moreover a simulation instability can appear if
position is exactly equal to one of the bounds defined by the
maxStop fields at the simulation startup.
Concerning the backlash, to begin with I would try disabling it entirely (set it to 0) and isolate whether the problem comes from that or something else.
Also keep in mind that the timestep matters a lot when activating the backlash. If it’s too big, it could be provoking these sort of instabilities. I wouldn’t go above 8 [ms] for the backlash values you’re mentioning.
Thanks for pointing out the discrepancy in the documentation, I updated it.