If you use the high resolution pack, try this:
- go to ..\Horizons\resources_override\interface\maps\Ista ria
- open map.def with notepad
- change the following line:
toCode:float MetersToPixels = 4.0
- save the fileCode:float MetersToPixels = 8.0
I'm not 100% sure it will fix it for you, but it did it for me when my map looked like a puzzle after some customisations.