Problem solved, it seems that there is an unknown (to me) declaration of LoadFont in the wrapper, so I changed it on the wrapper and all works now (last headers 1.3b released by klepto2):
File: /Leadwerks/Leadwerks.cs, Line 991:
//public static extern IntPtr LoadFont(string _name, int _size, int _flags, int _charset);
public static extern IntPtr LoadFont(string _name);
File: /Leadwerks/Font.cs, Line 19:
//return new Font(Core.LoadFont(path, size, flags, charset));
return new Font(Core.LoadFont(path));
So the call to Font.Load now requires only FontPath as it is documented in the engine.