Bspline FFD registration

Hi there

So I’m registering 2 lung CT images of same subject at different time points using the Bspline FFD approach . These lung images are in .mha format and I’m using the same approach as done in the POPI model.

So after iterating my output looks like this where I’m getting a rounding error .
Optimizer’s stopping condition, A rounding error occurred or line-search steps have an insufficient reduction
Iteration: 50
Metric value: 14402.493636195337

My optimizer conditions are as follows
registration_method.SetOptimizerAsLBFGS2(solutionAccuracy=1e-4, numberOfIterations=100, deltaConvergenceTolerance=0.01)

Can you provide some insights of what might be causing this .

Thank you