Choose the default "rotation mode" of the bones (Euler / Quaternion) — Right-Click Select
animation - Restricting rotational movement on my model? - Blender Stack Exchange
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Blender Tip: Rotation Modes Explained - YouTube
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How can I get a rotation from the head of the bones in pose mode? They rotate from the 3D cursor and I didn't find any option to get a normal rotation. :
rigging - How to AddDriver the rotation value of the armature for the rotation value of the object - Blender Stack Exchange
Rotating armature - Animation and Rigging - Blender Artists Community
Fix Cannot Rotate or Scale in Object Mode in blender 2.8 - YouTube