Thank you, Andi, for the reply.
I reinstalled the Sibelius Sound Sets, and I reinstalled the Plucked Instruments collection. (Both were reinstalled using Vienna Assistant.)
Still, the slur notations result in notes being played etouffe for the Concert Guitar. I can see this occurring in Synchron Player. “Normal” notes appear in Synchron Player as Long notes, Ringing, No vibrato, Sustain. The slurred notes appear as Long specials, Ringing, Etouffe. So the slur appears to be triggering this change for whatever reason.
Generally, slurs seem to work correctly with other instruments. Interestingly, for other instruments in the Plucked Instruments collection, slurs seem to behave somewhat unexpectedly. For the Upright Bass, the slurs seem to have no effect (no change in Synchron Player). For the Overdrive Guitar, they seem to result in an extreme vibrato with harmonics (Synchron Player shows Harmonics, Normal, Artificial fifth), rather than a slur.
Thanks for any additional help!
Best,
Michael