Steps to reproduce:
1. Open QA attached project "ParticleSystemShapeError.zip"
2. Open Test scene
3. Enter play mode
4. Observe Error thrown:
Assertion failed: Assertion failed on expression: '!"Shape not implemented."'

Expected: Particle system throws particles in the set shape
Actual: Particle system throws error

Reproduced on 2017.1.0b4, 2017.1.0b6 , 2017.2.0a1
Regression introduced on: 2017.1.0b2
Doesn't reproduce on 5.5.3p4, 5.6.1p1, 2017.1.0b1

Notes: It seems that during the update to the new version the shape of the particle system gets corrupted. On older versions it gets loaded properly.
You can fix the error by going to the "Shape" property of the Player Ball -> SubEmitterPUFF particle system and setting the Shape to anything else than the Sphere. After that you can set it to whatever, it's going to work now.

Repro project BEFORE 2017.1.x upgrade: https://oc.unity3d.com/index.php/s/99pq6A0vTIbRKpC
Steps to repro:
1. Open on 2017.1.0b2 and up
2. Open scene->level0
3. Press play

(richardkettlewell) UPDATE: These repro steps do not work for me. Instead of pressing Play, I had to instead select the Player Ball Game Object in the Hierarchy view.