@flashi Update: I did find an older save in Load save. Did everything as you said and it worked! Thank you so much!

@flashi Thank you, however the problem is I have no recent save prior to this quest. And there is no way to restart it. I could only try doing what you suggested from the 3rd step and see if that works. If you have any other suggestions, I'm all ears!

After interacting with all the clues and and successfully getting the information from the Druid in Druid's Cottage (Glowecestershire), the quest does not complete itself. Only the name of the quest remains (Clues and Riddles) while all the objectives are fulfilled. I tried exiting the game, saving manually and fast-travelling elsewhere witho no success.This bug makes it impossible to proceed with the Glowecestershire arc.