Nope - thats how ArchiCAD does things. Its triangulation of geometry on both Import & Export has always been terrible IMO, particularly the former.
We have a library of objects in C4D/3DS formats as they are the only real way to get decent looking objects - very little of what is in the ArchiCAD libraries ever gets used. It would have to have one of the ugliest collection of furniture, applicances, etc I have ever seen. However bringing 3DS files into ArchiCAD never works well, there is always horrible triangulation, unwanted lines, errors, etc. Anyway I have complained about this at length many times in the past so i will shut up now ...
As for your problem - if you are using the ArchiCAD Exchange plugin then I assume you are simply exporting the model to C4D to render?
These lines should not be a problem then .. they are just polygon edges and will not appear in your renders. I don't know why AC manages to turn a single flat surface into a bunch of triangles, but i've never had a problem with them when rendering.
They do make working with surfaces in C4D a bit of a pain though
cheers,
Owen Sharp
Design Technology Manager
fjmt | francis-jones morehen thorp
iMac 27" i7 2.93Ghz | 32GB RAM | OS 10.10 | Since AC5