[scribus] Small triangles when gradient diagonally

[Rolf-Werner Eilert]

I'm designing a Roll-Up with Scribus, and there is a slight gray 
gradient I have placed in a text box on the lower-most level.

When producing a PDF for preview, on my system there appear 2 white 
triangles (upper right and lower left) in this box, i. e. you can see 
how the diagonal gradient is produced by circling its graphical box by 
45°. In Scribus I cannot see this, it appears in the PDF only.

Obviously, the width isn't adapted enough, so it doesn't fill the whole 
text box.

Thank you for your advice.

Rolf