I finally tracked down the show and the particular episode that scared me the most as a child, shown on Nickelodeon.  Grimm’s Fairy Tale Classics, The Shoes That Were Danced to Pieces.  The link goes to the German version since I couldn’t find a copy in English.  There are so many elements here that freaked me out as a child:

1. Three princesses go down a trapdoor in the middle of their bedroom floor at night, wandering off in a drugged-out state, unaware of what’s going on.
2. They go down a really long flight of stairs and pass through a beautiful forest of silver, gold, and diamond trees, but it’s all surrounded by darkness.
3. They cross a river and enter a castle, where they dance with handsome princes.  However, partway through the dance, the princes change and are suddenly horrible demon monsters:
   

   Grimm's Fairy Tale Classics - The Shoes That Were Danced to Pieces

4. The princesses’ savior who followed them under an invisibility cloak breaks some stuff and gets the demons’ attention. The girls break out of their trances and they all run for their lives. What follows is a chase scene back across the river and through the woods, but the pretty trees have been replaced by dark, scary trees covered in snakes.
5. The princesses and their rescuer make it back to the stairs, still being chased by the demons, and of course someone has to trip while running up the stairs. The girls make it back up through the trapdoor, but the rescuer is stuck fighting off the demons. He gets away from them, but now the stairs are receding down away from the trapdoor, so his means of escape is disappearing. He has to jump and the princesses just barely catch him, hauling him back up into their bedroom.
6. Even once everyone is back in the bedroom and the trapdoor is closed, they aren’t safe. There are thumps on the trapdoor and they have to force it to stay in place, lest the demons break through and take them again.

So let’s see, we have helpless women wandering off in the dark, unaware of what they’re doing; crossing long distances and going far from home; humans turning into monsters; dawning awareness of your horrifying situation; a pretty forest suddenly turning scary; being chased by monsters through the now-scary forest filled with snakes; fighting monsters; being almost too late to make it to safety; and once reaching a seemingly safe place, finding it’s not actually completely safe after all. Yeah, pretty scary stuff for a six-year-old or however old I was when I saw this on TV. I think several episodes of Grimm’s Fairy Tale Classics were scary (see Bluebeard), but in particular I remember this episode scaring the bejeesus out of me.