Style a point layer in MapServer 7.0.4

I have a very simple problem with MapServer 7.0.4.

I am trying to display a point layer (WMS) with a simple circle but I can't get it to work.

Here is the CLASS definition in the layer object:

  CLASS

    STYLE
      SYMBOL "circle"
      SIZE 16
      COLOR 255 0 0

      OUTLINECOLOR 0 0 0
    END #STYLE

  END #CLASS

Here is the symbol definition, in a separate file called 'circle'

SYMBOL
  TYPE ellipse
  POINTS 1 1 END
  NAME "circle"

END

Here is the error from the log file:

[Fri Jun 16 09:47:51 2017].804000 readImage(): General error message. unsupported pixmap format
[Fri Jun 16 09:47:51 2017].804000 msDrawMap(): Image handling error. Failed to draw layer named 'sommets'.