Seems I am not the only one with this bug.
Edit: False alert!
I just waited some minutes more and now I see the nether world. Now statistically it happened only 90% of the time. ;-)
End of edit
Oh, here is a minor suggestion: Add this stuff to the group "not_in_creative_inventory"=1:
- nether:nether_portal
- nether:nether_torch_bottom
- nether:lava_flowing
This will remove these things from the creative inventory. These items shouldn't be placed manually, especially the portal tile; I guess it would only lead to problems. The other item says it even in it's description that it is not intended for manual placement. ;-) nether:lava_flowing behaves strange if you place it manually; probably only nether:lava_source should EVER be placed manually, not the "flowing" counterpart.