What is the resolution of your raster effect? Even if Illustrator creates vectors, the raster effect are not vectors. You have either to low raster effect settings in InDesign, or you have scaled the image in InDesign or you have used an EPS (which should not be used with InDesign) instead of an AI or PDF/X-4 file.
When you work with Illustrator, don't use PNG for InDesign, use AI or PDF/X-4. How comes your PNG into the game?
Personally, I would only save the trumpet, without the glow in Illustrator, import only the vector into InDesign and apply outer glow as effect to the imported file as it would give you more flexibility.
thank you very much! I didn't know that effects are raster images. At the time I added the glow effect to it I had already forgotten, that I had scaled the object way up in InDesign. Now it all makes sense to me.
The PNG was merely a test to determine whether it's a graphical problem of InDesign or if it would show in the export as well.
But it seems that was a problem of my workflow, rather the of InDesign! ;-)
Thank You very much!