- Regarding the "MeshInstance" error:
The Godot engine couldn't find the node "MeshInstance", because, in the code, the function "aim()", that responds for camera movement, tried to find a mesh called "MeshInstance", which does not exist in "Human - Player" scene. "MeshInstance" is the mesh name of old capsule-shaped "Player" scene.
I solved this bug, by replacing "MeshInstance" with "Armature", which has as a child the mesh for the new human-shaped scene.
- Regarding errors from the top:
As you said, it was a problem with signals. I figured out that when I copied the code from the old "Player" scene to the new "Human - Player" scene, there were some functions that linked the tree trunk areas (Node type: area), that allows the player to climb the trees, with the "Player" scene. And these functions really have nothing to do with the new player scene.
I solved this problem by removing the old signal functions and creating new ones, that link the tree trunk areas with the current player scene.
Thank you for your great troubleshooting suggestions!