How to setup reasonable ragdoll constraints between body parts? I'm trying to create a ragdoll to animate a falling person, following How To Make Any Rig Into a Rag Doll in Blender (Tutorial).
Basically, it means that you create a cube (rigid body) for each bone, and the connection between the bones is converted to an empty with a rigid body constraint that is set between every two cubes.
E.g. this is how my ragdoll looks like:

Note: The ragdoll is human size and 2m above the plane (so should fall once physics kicks in).
The second frame looks like this:

It looks like the ragdoll was hit by something really bad, not just obey gravity (what it should do) - the head is pulled backwards and the spine really breaks.
Here's a zoomed view of the spine area:

The highlighted object is the empty, and it's setup so that:

It has a generic physics body constraint.
There's a zero limit on the distance. This (I belive) means the two spine cubes cannot change distance from the empty - it should act as a hinge.
There's a bit of freedom in changing the angle (between objects?) - represents the common back of a person (non contortionist).

Why is there such a big change from the 1st frame to the second?
Note: I also tried with no angular limits (thinking the limits are outside the initial rotation and might cause an abrupt change), but it didn't seem to help.
For completeness, here's the .blend file for this.
Thanks.
FWIW, the simple answer for my question is that the constraints between objects are fine as they are.
There is some physics force or bug (yet to be determined) that causes this to happen. Why? Because if I move the whole ragdoll to somewhere else, this doesn't happen.
Added a followup question trying to identify what's that mysterious force, but the ragdoll &quot;wiring&quot; seems fine.