No rounded face possible


I just did a refinement of my model, which was loaded as STL file, that got exported from Siemens NX CAD system.
After I deleted all doubled vertices I still get this strange behaviour of the rounded face.
See at the ends of the rounded face in the screenshot.
I am stuck with optimizing…what can I do here?
What goes wrong?

Thanks for the help.
Cheers from Germany


Use quads and triangles only
Physically look at the mesh at these ‘artifacts’ to check mesh integrity

For specific assistance please supply a blend file with the object

Thanks for the answers.
The same question brings some additional answers on stackexchange:

No offending intended, just for comleteness.
Will look for the best workflow and results and post this here as well.