I recently completed this quest on a second character and noted that a hint in the guide was misleading. It indicated you could use Magic in the final battle against the image of yourself. I equipped enough runes for 30 casts of fire blast and some food in case the spawn was a higher level than I. The sequence would not load. I tried various combinations of inventory each met with failure until I went in with no Inventory at all.

 

It is possivle that the Seal of Passage might be allowed in, the last inventory i tried that was rejected was just the Seal and a tinderbox.

 

Problematic to not having the Seal when i finished and attemptted to talk to the NPC for completion I was ejected from the island.

As far as I can tell this is a bug. I did the quest the other night without incident.