Hi JuliusD, I had the same problem with the disconnecting textures and it was driving me crazy! I couldn’t find a fix for that, but for the problem you have with the lightmap I could. What I did was delete all the meshes that had lightmap problems, and in 3Ds max I created an Unwrap UVW modifier for channel 2 (leaving my texture mapping intact) and then flattened it. Then I re-imported this meshed via Datasamith, and in the **Static Mesh Build Settings I **used the new UV (in my case 1, as 0 is the texture one) as the source lightmap index and 3 (the UVs automatically created by UE) as the destination lightmap index. I also set 3 as the lightmap coordinate index.
This fixed my black faces problem.
Hope it helps!