A morning coffee and I love this place! If there is not an immediate solution, at least one is pointed into the right direction! Thanks to you two!
@Tarot: I know that glass tutorial. It's not using an additional texture for the glass - only a material, but I looked into it again and found a helpful environment note.
@Zwerkules: That was a good tip. I tested some more settings, and I think, I figured out a working solution (see below). I tried to locate the Rolo-post, but I failed. If someone manages to find it, I'd be glad to understand this a bit more.
This environent thing with placeables was driving me nuts! At one point, I had a glass + env.-reflection - also IG - and when I added another one with the same settings, the effect was gone, no matter what I tried, but it stayed on the first placed placeable... It makes a big difference, how the alpha and the env. is handeled, by setting the placeable to static or not static (as Zwerkules wrote).
Anyways, this way it seems to be working:
For the AuroraTrimesh modifier aktivate Over-ride Mat Values. I used the default settings here. I did not do that before and I had these strange "works once" happenings (see above).
In the placeable.2da set "Reflection" to "default" and "Static" to "1"
In the toolset create a placeable blueprint and make sure, that "Static" is NOT checked!