don't explicitely set the font names, it is default anyway and confuses dot because being not quoted anyway.
Thanks bastian.germann@gmx.de for the hint leading to this

diff --git a/doc/full.cfg b/doc/full.cfg
index d3d864a..88bab69 100644
--- a/doc/full.cfg
+++ b/doc/full.cfg
@@ -1470,29 +1470,11 @@ HAVE_DOT               = NO
 
 DOT_NUM_THREADS        = 0
 
-# By default doxygen will write a font called FreeSans.ttf to the output
-# directory and reference it in all dot files that doxygen generates. This
-# font does not include all possible unicode characters however, so when you need
-# these (or just want a differently looking font) you can specify the font name
-# using DOT_FONTNAME. You need need to make sure dot is able to find the font,
-# which can be done by putting it in a standard location or by setting the
-# DOTFONTPATH environment variable or by setting DOT_FONTPATH to the directory
-# containing the font.
-
-DOT_FONTNAME           = FreeSans.ttf
-
 # The DOT_FONTSIZE tag can be used to set the size of the font of dot graphs.
 # The default size is 10pt.
 
 DOT_FONTSIZE           = 10
 
-# By default doxygen will tell dot to use the output directory to look for the
-# FreeSans.ttf font (which doxygen will put there itself). If you specify a
-# different font using DOT_FONTNAME you can set the path where dot
-# can find it using this tag.
-
-DOT_FONTPATH           =
-
 # If the CLASS_GRAPH and HAVE_DOT tags are set to YES then doxygen
 # will generate a graph for each documented class showing the direct and
 # indirect inheritance relations. Setting this tag to YES will force the
diff --git a/doc/public.cfg b/doc/public.cfg
index 90a0c33..3c887e4 100644
--- a/doc/public.cfg
+++ b/doc/public.cfg
@@ -1470,29 +1470,11 @@ HAVE_DOT               = YES
 
 DOT_NUM_THREADS        = 0
 
-# By default doxygen will write a font called FreeSans.ttf to the output
-# directory and reference it in all dot files that doxygen generates. This
-# font does not include all possible unicode characters however, so when you need
-# these (or just want a differently looking font) you can specify the font name
-# using DOT_FONTNAME. You need need to make sure dot is able to find the font,
-# which can be done by putting it in a standard location or by setting the
-# DOTFONTPATH environment variable or by setting DOT_FONTPATH to the directory
-# containing the font.
-
-DOT_FONTNAME           = FreeSans.ttf
-
 # The DOT_FONTSIZE tag can be used to set the size of the font of dot graphs.
 # The default size is 10pt.
 
 DOT_FONTSIZE           = 10
 
-# By default doxygen will tell dot to use the output directory to look for the
-# FreeSans.ttf font (which doxygen will put there itself). If you specify a
-# different font using DOT_FONTNAME you can set the path where dot
-# can find it using this tag.
-
-DOT_FONTPATH           =
-
 # If the CLASS_GRAPH and HAVE_DOT tags are set to YES then doxygen
 # will generate a graph for each documented class showing the direct and
 # indirect inheritance relations. Setting this tag to YES will force the