Numbering of heading 3 cannot be displayed

I’m using Libreoffice Writer 4.2.0.4 on Ubuntu 13.10 (64bit, English).
When I use the style “Heading 3” the numbering of existing documents will be displayed as rectangels. All other heading styles show the correct number. The style is set to Arial Regular 12pt.
The Numbering style is set to WW8Num2. Is there a way to change this property because it is greyed out ?

I could change the Numbering style of all heading styles to “None” but sometimes the complete numbering of the document will by destroyed. Is there a way to get the heading 3 numbering displayed correctly for existing documents without such a big effort?

The Numbering style is set to WW8Num2. Is there a way to change this property because it is greyed out ?

This usually indicates that outline numbering is in use. Try Tools > Outline Numbering…