I have two animations in my current project. I used the the graph editor for the second animation (frame 25 to 48) and can no longer insert keyframes behind the last frame of the second animation. Or rather, when I create a pose, mark all bones, and insert a new keyframe, it does insert a new keyframe, however the pose is instantly changed to the last keyframe of the second animation when I switch the current keyframe (I have also tried to insert a visual keyframe, without any changing result).
(Not sure how I can embed a video, so I uploaded it here): YouTube Video
In the video you can also see that the graph editor does change, but the animation doesn't.
I am not sure if this has something to do with the issue, but when I tried to identify the error I saw these weird keyframes which I can't remove. They don't seem to do anything (They were inside the actual keyframes and I discovered them when moving the keyframes)
I suppose I did something wrong with the graph editor but don't know what. The only thing I did inside the graph editor was to change the curve of the y axis of the feet in the walking animation.
EDIT: The Blender file here.