Andi,
I am using the subscriber version of Sibelius, so I have v. 8.4.1 installed. I am running OS X El Capitan 10.11.6. So in both cases I am using the latest official releases.
You guys are always great at fixing issues, so I have complete confidence you will figure out what the gltich is. I have worked the new Ensemble Pro 6 version (using VST) and I love it. But I'm glad I won't have to redo all my old configurations.
Randall