The underside of the Curb SubBase and the Urban sidewalk both use the Datum code. If you apply that code to your surface then the software doesn't know what to do where the two links overlap. In the corridor surfaces tab you can specify the overhang correction option which will help.
In this case if you set the overhang correction to bottom you're probably going to get a surface that runs diagonally from the outside corner of the subbase extension to the ouside corner of the sidewalk.
Steve Please use the Accept as Solution or Kudo buttons when appropriate