Some bugs have been reported about the 3DS exporter. Sometimes GMax does not want to read the created 3DS file. Tonight I looked into these issues a bit and it seems to be related by the length of some lists being defined as an unsigned 16 bit integer, which means they can contain a maximum of 65536 items. So if you have a part with more than 65536 triangles or vertices, than the created 3DS file will be invalid and gives the error. I am going to think of a fix for this issue, I'll probably have to split up parts that go over this limit. If you also have problems with 3DS files, but they are not so complex, please let me know. In that case there might be other bugs as well.