Have there ever been any accidents within holding stacks? At a busy airport, up to a dozen aircraft waiting to land may fly the same holding pattern at the same time, separated vertically by 1,000ft. &mdash; a stack. Some ATC towers have dedicated Stack Controllers.

It's all very routine, but have there ever been any instances of incidents or accidents in stacks?

To the layman, it might seem like stacks could be hazardous &mdash; e.g. what if a plane in the middle of a stack lost an engine?
The article you linked in your question mentions one incident - loss of separation - that's described in more detail here.

The Aviation Herald is a good source of information on incidents and accidents but it has very few reports related to holding. There are several about low-fuel incidents because aircraft had to remain in the hold for a long time (Avianca 52 is a well-known case), but I could only find one that involved a holding pattern more directly and that was also a loss of separation resolved via a TCAS advisory.

Eastern Airlines 401 crashed while in a hold, but it was an ad hoc holding pattern and not in a stack with other aircraft. American Eagle 4184 crashed due to icing accumulation while holding but again there were no other aircraft involved.

I could only find one example of an actual collision in a holding pattern which was over New York in 1960 and in that case only one of the aircraft was holding (inaccurately).