Question: If Ravenloft is a demiplane, and a demiplane is an extradimensional space, can spells that create extradimensional spaces (like *rope trick*) be cast while in the realm?

Answer: 
Yes. 

> 
>  The Mad Mage of Mount Baratok has *Mordenkainen's magnificent mansion* prepared and can successfully cast it. 
> 
> 
> 

The demiplane created by a spell like *Mordenkainen's magnificent mansion* (PHB. p. 261) still follows the same rules regarding planar travel, so while you can create and enter an extradimensional space, you can't then planeshift out of that extradimensional space to escape.
Is there any reason why you think it can't?

Rope Trick says:

> 
> At the 
>  upper end of the rope, an invisible entrance opens to an 
>  extradimensional space that lasts until the spell ends.
> 
> 
> 

There is no reason if, having entered that extradimensional space, that I couldn't cast rope trick *again* and climb into a different extradimensional space.

Since the DMG says of Demiplanes

> 
> Demiplanes are extradimensional spaces ...
> 
> 
> 

The same would apply.

Technically, all planes are "extradimensional spaces" the difference between Demi- and non Demi- one's is that the former are finite and the latter are infinite.