Thanks a lot for the tipp! I can't remember a music note on a tree trunk so that might be the case for me, too.
Update: Damn, it really was the same bug. The green sing stone on the tree trunk just wasn't there before restarting the game. Thanks again! I now finished the game and even figured out how to use the old key and what the small wooden tunnel is for. :) Finally able to finish the game.
Hi, a lot of people had trouble with the green stone. It's unlikely that it disappeared completely, but maybe you've pushed it behind something unknowingly while walking?
In any case, I've changed that specific part of the game in v1.3 to prevent such a thing from happening again. Read more: https://sokpop.itch.io/frogs-adventure/devlog/576568/v13-bug-fixes
Thanks for reporting the issue!