One leg won't move anymore, underlined red in DopeSheet

Hey Blender artists!

I’m doing an animation of a Colossus from Starcraft 2. I animated it and it was working great, then one of the legs is now not moving. In the Action and DopeSheet the name of the IK handle at the bottom of the foot which moves the leg is underlined in red.

I can just copy and past animation from one of the other lags, but i want to know why only one part of an armature stopped working at a seemingly random point.

Thanks for the infoz!

I got the same problem currently looking for an answer, funny thing is the curves are still there so the animation in theory should still be there, the bones just became uneditable, I cant add frames to the bones that have that red underline, and they don’t move anymore even tho they have animation frames applied to them.

Found a solution! inside the dope sheet on the left side where your bones are, shift select all the bones that have that red underline, then got to
Channel: Revive Disabled F-Curves, and the bones become editable again.

Now what puzzle me is that I never asked the app to disable those curves, I don’t have and idea as why they got disabled, but at least now I know how to get my animation back.
I gotta tell ya it took me hours to tweak that anim and i was getting pretty angry about this glitch, I hope it this helps you, lord knows I was loosing my mind with this problem.

Found an explanation for this behavior:

Animation Editors: Experimental indicators + “fixup” operator for “Disabled FCurves” F-Curves tagged as “disabled” now have their channels drawn with a red line underlining their names. “Disabled” F-Curves are skipped for evaluation, and typically result (for example) from assigning an action from one armature to another, but the new armature does not have some of the bones the action’s F-Curves need in order for the datapaths (referring to the property the F-Curves affect) to be resolved. This is to prevent heaps of invalid channels slowing down animation playback. I’ve also added a new operator, found by: Channels -> Revive Disabled F-Curves in the Graph Editor and DopeSheet/Action Editors, which will clear all the disabled tags for all the F-Curves in the animation editor at the time (based on the filtering criteria). Use this operator to clear the disabled tags, allowing such channels to be able to be evaluated again (perhaps after adding the offending bones for example, or when using the action on the original armature again).