I have a similar issue with the same error message. I’ll go through what I was working on before getting this.
I was working on an action and was using a video as a reference. I thought I could get fancy and use tracking to guide the bones, but it didn’t turn out too well so I just made keyframes by hand. (might not be the problem but different than what I normally do)
I may have added more non deforming bones during the animation.
The prime suspect seems to be a broken nla strip. The “Rig” object has the “Walk” animation listed under in green. It’s not movable strip, and not the current animation being edited. I know of the tweak modes when pressing tab which doesn’t do anything special to this. The panel you push n by default to see just has one dropdown for “Animation Data” where it also shows the walk animation and all it’s users I can’t find.
Some bugs that occur regarding the strip
-When I unlink the walk action it will add a new user.
-users will not show up in the outliner.
-No option to push down or stash into the nla.
-can’t export with binary or ascii with the armature.