Can't create new pose library or add new poses to existing library I have a file with several armatures, which I duplicated, and tweaked each for another mesh. At some point I couldn't add more pose libraries or add new poses to existing one. Did I lock something somewhere?

Thanks in advance.
Looking at the screenshot you are in the object mode of the armature.

You must be in the pose mode of the armature in order to enable the button for creating a new pose library and adding poses to the library.