Martin Gregorie wrote:
That is just an urban myth.

I've spun ours two up with normal CG position.

The manual is often misunderstood in this point. It states that with a
CofG of less than 315mm (=12.4 in) the ASK21 wold not spin at all under
any circumstance. With a CofG more aft it *may* be possible to make it
spin, however this depends a lot on the total mass, the mass
distribution and the spin entry technique, which can be tricky. (Exact
speed, nose not too high or too low, help with ailerons...) This can be
achieved without tail ballast. But only with a CofG aft of 400mm
(=15.75in) the spin can be reliably provoked with the standard method
and the resulting spins are reliably sustained, and this CofG cannot be
achieved without tail ballast. Hence the tail ballast for training.

The already mentioned report by the USAF (AFFTC-TR-89-27) states the same.

So with a CofG between 315 and 400 mm you are not "safe" of spins, but
the ASK is not suited for basic spin training either.