The right ramp, if you face the ziggurat, on the ziggurat's south face, cannot to used. It's the one earlier covered by rubble. I guess this has something to do with it. It's like if it's blocked by an invisible object.
Experienced this as well!
(http://i17.photobucket.com/albums/b83/staveguard/screenshots/efuinvisiblewall.jpg)
Definately the case.
There used to be a pile of rubble there that has disappeared into thin air!
And you could actually walk through the rubble. :/
Needs a fixing
newbs
use the other ramp
fixed