The Calydon Ruins Quest Bug

Was wondering if anyone can help me...

I am at the point of the quest where u go into the calydon encampment to get something from each of the 3 plaque stones... during my second stone, my inventory is full so the quest item could not enter my bag... but the quest still updated itself and i cannot loot the 2nd stone anymore... and now i cant continue the quest due to this bug...

Wonder if there's a fix for this bug...

 

Did you try clearing up your inventory? I've had items go into spots that didn't exist, so I couldn't see them until I cleared uip.

 

Clearing inventory up? how? thing is, if it's still in my inventory, when i reported the quest, the item should be identified as "exist in my bag" and yet the npc says that i still havent gotten all the item yet... despite the quest says it's completed already...