Had the same problem.(with UE 4.23)
I think Unreal engine changed DDS format.and spacescape has not been updated yet.
you have to open the file in photoshop (or anything like who can open DDS file) and re-save in DDS (32bits/Channel, 8.8.8.8 ARGB 32 bpp, unsigned) no mipmaps, and then you can import it to UE.
hope it works for you