** Warning 1902027 ** There is one-sided surface found near element %d and element %d.
A one-sided surface means that following the connected elements, there is no unique surface normal direction for an element. This means that the midplane trianglular elements must have a twisted connection.
Select Mesh Mesh Diagnostics Orientation Diagnostic to show the mesh orientation and locate the two elements indicated by the message. You should then decide whether a cut should be made somewhere to make the element normal unique.