I finally found a solution to the problem with the missing Lightmap UV channel when exporting fbx from cinema 4D.
Here’s how you do it to get two proper UV channels out:
For every object in your scene, you have to create two UVW Tags. You can simply do that by applying a material to it, select the projection you would like in the material tag (cubic, or spherical, or cylindrical NOT UVW, this will not work)
and then (with the material tag selected) click on "Tags/Generate UVW coordinates.
The second UVW Tag can simply be added by applying a second material to it and just repeat the process.
Now comes the tricky part: In order for U4 to recognize the two channels you need to rename the Tags.
Click on the UVW Tags, and under Basic/Name where it is written “UVW” change this to “1” and int the other tag to “2”.
U4 will use Tag number “2” as Lighting map by default, so make sure it has no overlapping UV’s.
You can do this by going into “BP uv edit” mode, select your UV tag and click on “Show UV mesh” in the UV Mesh menu on top on the right side.
Then select “UV Polygons” on top in the middle and now you should see your UV map in the editor on the right. With CTRL+A you can select all Polygons and now you should be able to play around with the “optimal mapping” settings underneath.
Do this untill you have a proper UV map with no overlapping polygons anymore.
For Tag “1” it is important to have a UV that fits to the shape of the object (because you wonna apply materials to it later in U4), so spherical for Spheres, cubic for cubes etc. In my experience it does not matter for Tag “1” wehter you have ovelapping UVS or not. Sou you don’t neceserely have to go into UV edit for that.
Now for exporting the whole scene into U4:
First, select all your objects and put them in a group. Then export the fbx and import it to U4. Do NOT check “combine meshes” this will put all your objects into one and also overlap all your UV Tags into one, and therefore you will get the “overlapping UVS” error when building the light.
Now, you have all your Meshes in the content browser, how to get them all at once in the editor and keep the relative coordinates?
Well it is a bit buggy. Sometimes you have to try 2/3 times untill it works. Just select them all and drag and drop them in your editor. If it does not work, and your objects are misplace, delete them and play around with your camera view in the editor, sometimes it helps if you are far away from the place where you import your fbx meshes. Honestly I don’t now why it works sometimes and sometimes it will not, you just have to try…
If it works and you have all your objects at the right place, I suggest to select them all, check “overriden Light map Res” in the Lighting settings and raise the Number up to at least 200 to get some nice shadows.
click on “build” and if everything went well, you should have no error message about any UV’s anymore.
Hope this could help some of you struggling with this