Can't hear sound through headphones; software glitchy

I am new to Pianoteq so please bear with me.

After some initial difficulties, I successfully loaded Stage onto my MacBook Pro and did the following hookup: Roland fp-90 to MacBook Pro via USB cable; Macbook Pro to Schitt Asgard III headphone amp. I use high-quality Beyerdynamic headphones. Everything worked fine until the other day when I could no longer hear Pianoteq through my headphones. Sound would emanate from the laptop speakers despite "output" being set to "external headphones." But the problem has gotten worse. Now, when I select "external headphones," the software immediately goes into spinning disk mode, completely in computer purgatory until I force-quit the program. Also, I occasionally can't open Pianoteq at all. The screen icon bounces up and down and the software doesn't open. For what it's worth, this problem didn't surface until I encountered the problem with selecting external headphones.

Does anyone know of a fix? Should I uninstall and reinstall the software? Is there a particular trick to doing that?

Thanks for any assistance you can offer.