Saving is disabled when the game thinks it is in a cut scene. Can you save immediately after entering, before taking a couple steps to trigger the cut scene conversation with the sleeping guard?
See;
Game crash upon going to Verdistis and
Game crashes when I try to return to Verdistis.
BTW, have you installed the latest
patch?
You do not actually need to join the thieves' guild to find the castle treasure room sewer entrance; it just gets flagged on your map if do.
