Importing a custom animation always saves the animation path as RootNode/Armature/Skeleton3D. When adding the animation to my AnimationPlayer, I have to manually edit out the RootNode of each entry, as my character set up is just Armature/Skeleton3D.
I don’t think mass renaming is supported in the animation player editor.
I don’t know what may be causing the root issue (wrong animation path) but you may want to check your model source file to see if it’s correctly setup and any modifications have been correctly applied.
If you think everything is setup correctly and it’s still failing to import you may want to open an issue here Issues · godotengine/godot · GitHub attaching the model in question and the problem you are having so the development team is aware of that issues.