Sorry. Unfortunately I've had no luck reproducing any error, I save the drawing from v17 to dxf without altering it, opened in in Trueview (and Autocad program), TC v18 and Corel Draw, all three opened the dxf just fine.
Hopefully someone who may have seen this problem before, and can commenton the cause, all I can suggest is try the following and see what happens.
Select the polyline fills (not the polyline outline), right click, select properties, under the brush section, click where it says by pen, and after the colours is an icon for 'by layer', you could try setting it to that, in case the other program is having problems with this setting, or you could just set it to black.
You could also try in the same dialog changing where it says 'alternate' to 'winding' but I can't see how this would make any difference - just clutching at straws really as I can't reproduce any problem.
##Edit## there is a setting in the dxf save dialog called 'preserve hatch Associativity' maybe someone can comment if this setting can cause problems (I don't know what it does) ##End edit##