I am sorry you are having this issue. From what you have described you have created a BSP inside of you scene and then converted that BSP into a static mesh. You then build lighting and the static mesh turns black.
If you would link me a screenshot of your static meshes details panel. You can double click on the static mesh in your Content Browser to bring this up.
BSP’s are calculated differently, when it comes to light map calculations. Typically a BSP has a density of 4. Therefore, whenever you convert your BSP into a static mesh the Lightmap Resolution persists and returns a value of 4. Increasing this to values like 256 or 512 should fix this issue because now you are calculating at a size that is reasonable for a Static Mesh.
Try this out and let me know if this fixes the issue. If it does not then please post screenshots, as mentioned above, and I will continue to investigate this problem at that time.
Sorry to hear you are still having a problem with this. I did a quick test by converting a BSP to a static mesh and this method seemed to fix the issue.
In order for me to test this in completely the same way as you I will need a few files. If you would link me your .uproject, content, and config folders for the project you are having this problem in, then I can attempt to recreate this issue.
I just made a new level, deleted the start block and placed a BSP with the size 500x500x40 and converted it to a static mesh after which I build the light and it was black, I changed the min lightmap resolution to 256, it didn’t change anything, then I changed the lightmap resolution in static mesh sessions to 256 as well and it still didn’t change. I also changed the UV channel to 0 and 1, but there was no difference. I have also deleted the intermediate folder multiple times with no change.
I updated to 4.9 btw and the issue is still there.
I believe I have solved your issue. Whenever you are in your details panel of your BSP, scroll down to your Static Mesh Settings. By default there is a tab with an arrow pointing down. Click that. There is a tab for Light Map Coordinate Index tab. Change that from 0 to 1.
Still issue in 4.13 and setting Light Map Coordinate Index to 1 fixes it. In 4.12.5 the same Index bug was causing Lighting build issue (overlapping UVs…).