Hello Werner,
Thank you for your interest using the LM25088. For the Inverting Buck Boost design in WEBENCH, it looks to me there is a bug within the software prevents you to optimize the design. We will investigate the issue.
For now, the only option for optimizing the circuit is using the Knob setting on the top left corner of page to adjust for freq. or price or footprint size.
I apologized for the inconvenience and we hope to fix the old code as soon as possible and will let you.