How can I edit greyed out Keyframes again? I'm having some issues that some keyframes in the Action Editor are greyed out and I can't edit them anymore. I'm not sure if I accidentally pressed some button, but this has never happened to me before and I've been using blender for some years now. 
MAIN PROBLEM:
I can't delet the keyframes, set new ones or do anything, not even more them on the timeline. Anyone having an idea what is wrong?
You have Locked your keyframes. Press the padlock icon next to the object name to unlock them again: