I've attached some screenshots of what I mean here - the tail, back, and neck area all have IK Spline Handles on them, but in the example, I move the back cluster and the neck jitters around wildly, but everything else - tail, legs, etc. seem to act normal.

Even when I undo moving anything (including the back or other chains), the neck will stay in an awkward position instead of going back to its starting position. I have to use "Assume Preferred Angle" for it to go back to normal.

Is there anything I can do to remedy this without having to redo my neck and constraints?

Also, the error is shown in the error window thingy on the 2 - 4 images.

Thank you!!