Treasure located!!
Update, June 12: Unfortunately, the treasure is not there any more. I checked it today, and the whole box including the remaining chocolate and the logbook is gone. I am very sorry, someone must have taken it :-(Congratulations, you solved the riddle!
The treasure WAS located at
N 48 31.478
E 9 04.487
Enjoy your pentecost break!
Spoiler images:
