Ok fixed it. I had to make one surface out of the 4 polygons. Before it was two surfaces and only one vertexmap. No problem in LW, but the converter didn't like it. After I made this I had the duplicate surface-name problem you adviced to me.
I have renamed the surface and voila the #%&*$# converter is doing it's work!
. Oh man, who has excogitated such a screwballed approach.
There is no way to simply remove an unused surface, you always have to rename it and after this start the modeller again, to save the now corrected model (surface removed). Slowly I remember all those things again, I also hated back those days.
Another problem is that all my plugins I used aren't available any more, cause I had to reinstall my laptop. I had collected a lot of plugins which made things possible LW isn't able to do the easy way.
I hope that all is functional after the surface rename. I have the vague memory, that if you rename a surface, LW also renames other surfaces, which could result in things like non functional MFD's etc. If the surface-name of the surface which displays something is changed, the whole thing isn't working anymore. Could remember something similar back those days, which costs me endless nights to fix it again