There's a bug in this game where a character won't let you pass even if you've successfully completed their minigame. There is a "fix" for this:

When the player is at the very bottom of the tree, extend the player vertically and a button "alle spiele freigeben" should become visible. This button will cause all characters to move aside, letting you move freely up the tree without the need for minigames. If you still want to play the minigames, you can click on each character on the tree in the main menu.