Heya MadMusicologist,
Wonder if copying a dll to a Sibelius folder might cause the issue.
After installing Pianoteq, I've never needed to 'copy a dll' somewhere. Install is all.
(*you may have reasons - but it does seem the likely issue, based on the info provided).
I point to it, because I use various DAWS and hosts/OSs and over time, I've never had to do anything beyond installing Pianoteq, to have it run actually very close to 100% error free everywhere (as standalone, or as a plugin in any DAW or host).
Instead of manually copying any 'dll' to a special custom locale, is it alternatively possible within Sibelius settings to create a path to where all VST's install to? (like the Steinberg and/or other common directories?).
Or, whilst installing, is it possible to tell Pianoteq to install the plugin path as the custom one you want?
[For the many times I've updated Pianoteq, I actually cannot just now remember what path information it asks for when installing - because, I have never had to jink things in custom ways.. I've pressed "yes" during installs.. this is how I prefer, and with audio applications, to try to make all programs use "well worn paths" - honestly, this does seem to have improved in the last half dozen years to a decade. I now feel there are, at least for me (with a great many audio tools), less and less valid reasons to customize disperate folders for multiple, potentially clashing installed audio 'extensions'.]
I get very nervous, when a plugin warns me "Be sure to select the correct path" on install.
Keeping things as standard as possible, rather than a jungle of custom paths (for reasons) has turned out the winning way, at least for me.
Hope that gets it, but let us know - and good luck with that music.
Pianoteq Studio Bundle (Pro plus all instruments) - Kawai MP11 digital piano - Yamaha HS8 monitors