sry for the delay, had a mean deadline yesterday…
i’ll post the solution i endet up using here, maybe it will help someone in the future:
so in blender we set all textures as emissive, then in the loader-extention i iterated over all texture-names, loading a .png file if there was the word shadow in the name and setting it as diffuseTexture with alpha. Otherwise i used the .basis file as emissiveTexture… It feels like a bit of a hacky solution, but it ended up working 
If someone knows a better solution i’d still be interested
apparently .basis (or the babylon-basis-interpreter) can’t handle transparencies? (or at least i allways got an error when i tried to load a .basis file that was created using a .png with alpha…)
@labris: thx, i’ll have a look at the tool you recommended. I’m not super satisfied with the result i got from BinomialLLC-converter, since all the clay-render textures i used it on got some kind os redish flair in the highlights, that isn’t there in the original png. (But this is maybe a topic for another post:)
@bghgary: Thanks for your reply. Yes i do (and it works
correct me if i’m wrong, but isn’t .gltf just fancy json, only storing paths and dependencies?
in my playground, there was an old test-texture missing, i fixed it now. But the problem remains the same: when trying to load .basis-textures created from .png with alpha the loader crashes. (if one removes lines 87-89 in the playground it works fine - not loading these textures)
in the meantime i found a different post with a similar problem, but i only unterstand half of it…
i’ll have to do some more research on the compressed textures topic obviously. i’m completly new to this…
thank’s everyone