I recently upgraded to MacOS Monterey and am shaking out the bugs. Not clear whether this is one of those.
Create a new Logic Pro X project. Add Great Rieger Organ as an "AU" instrument on a track. Check that it plays (command-K). It does play.
Try to open the plugin to select (organ) stops. Opens a window that has "Vienna Organ Player" at the bottom, but otherwise empty (black background). That is the error. I cannot select stops or otherwise configure the organ in the plug-in. In the past I was able to configure the organ in this window.
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On the other hand, the standalone app Vienna Organ Player can open the organ, I can select stops, and I can play it in the Player.
I also have VSL and I can create tracks that use the VSL instruments, which play perfectly and show up in the plug-in interface (for instrument selection).
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I am using the eLicenser and it shows that I have VSL plus the Great Reiger Organ licensed.
(In the Vienna Assistant it says I have no licensed products. I believe this is a known bug.)
I notice the VST plug-ins are from mid-2021 and I wonder if there is a way to update those without having to delete and re-download the whole organ.