Hi iacdxb,
Thanks for the Dropbox file. The Forum’s attachment system is completely broken at the moment. So I couldn’t open your file at all.
My suggestion, use the Extrude Function, while the Create Cap is checked.
https://help.maxon.net/us/#TOOLEXTRUDE-MDATA_MAINGROUP
Possible problems: the checkbox (Create Caps) is left on for another Extrude on the same object, which creates internal polygons and destroys so any use with the SDS (SubdivisionSurface) for good. Have an eye on that!
All the best