Unstable configuration detected: returning points to original positions

I am attempting to do an Adjust Track. I have a shot of a road sign from a moving car where I’m going to replace the image on the sign. I’ve tracked with the spline tool, and now I’m using the tangent tool to set up the corner points of the sign. Its working, although a lot of manual adjustments are required due to changing light condiitons. I’m fine tuning, but now, when I try to adjust a corner point, I get the above mentioned error, and I am unable to adjust the point.
Can anybody tell me what this error means, and what I’m doing wrong?

How far are you moving the adjust points from the original surface?
Are you using “auto” or manually adjusting?