Problems Modifying Keyframes in Masks.

In my After Effects project, there are some layers that include simple masks, the positions of which are animated.

The keyframes I had created to control the mask animation had been working fine. I had been able to change the layers, including their keyframes without issue. But, now, for some reason I don’t know, AE’s behavior has suddenly turned erratic and it will not allow me to modify many of the keyframes in the masks. I can’t select or move those keyframes, even though their respective layers do not appear to be locked.

And, when I try to modify the keyframes in the graphic editor (to move them or create easy in or easy out, etc.), I now receive an error mesage that reads “Attempt to get an item address from a position outside of list length. ( 27 ::15 ).” When I click OK to clear the error box, the same error automatically reappears over and over; the error will not clear.

What is happening and what can I do to modify the keyframes properly again in AE?