[Radek]

In my experience, you can create portal anywhere in the game except the Pyramid. The portal will work and the Teleport spell will take you to your portal from anywhere on the current map. The Pyramid is the only place in the game where you cannot create a portal and, of course, where any Teleport spell will fail.
On the other hand, there are places in the game where you have to expect the "teleport bug". One such place is the south shore of the sea behind the Anephas Shrine. But you can create a working portal even there.
Perhaps, there were planned places in the game, where you weren't allowed to create a portal. Perhaps, these places were removed from the game later. Perhaps, the "teleport bug" is a result of incomplete removing these places from the game. I don't know.