Torpedo Duomatic "sudden brake", then heavy to get loose

Hello again guys,

today there was a guy with a Torpedo Duomatic
and I recall I've seen his problem before, also
on a Duomatic.

When you brake, instead of getting a modular
effect, at some point the break virtually looks
the wheel - if the bike is upside down, this
happens all but instantly!

After this, the wheel stays like that unless
considerable effort from the crank will get it
loose and one can start pedaling
normally again.

Here is an explosion of the Duomatic
rear hub [1]. It is in German but I suppose the
world of mechanics has a language of its own.
The brake shell is part 11, right?

Anyone knows what to do/replace?

TIA

[1] http://user.it.uu.se/~embe8573/bike/duomatic.png

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