Please go to Help > Create Diagnostics Report, that will create a zip file on your desktop. Please send it to ‘u dot stoermer at steinberg dot de’ and also tell me the path of where your Sonatina plug-in lives. I’ll then adjust the path file and send it to you.
I could help Marc and he has Sonatina Choir now running.
Thanks Ulf, I was about to write it ! I didn’t have the plug in installed at the right place (there’s no installer with it). In case you have a mac, it should go in Library/Audio/VST folder. I tried in my user library with no success, but in the library it did work. I whitelisted it writing Sonatina Choir and that’s all.
For Derrek : I have bought NotePerformer and it is much more interesting. Sonatina is a nice alternative to the horrible Aah to Ooh from HALion (everybody seem to agree on the fact it is unusable), but not sure it’s better than the plain Aah from HALion. The good thing is that it is free.
My choral problems were solved with Korg Choir Aahs, a free sfz extracted from an older soundfont. Straight-tone English cathedral choir with young treble voices and countertenors!
It’s not the solution for everything, but for pure choral fussiness it’s important to have around. I blend it with Garritan’s choir voices or Olympus Choir Micro (using a Kontakt Player instance rather than Aria in one bank) when I need a massed choir. (In truth, mixing them up a little is how actual choirs and string sections work - a range of imperfections is what creates choral perfection.)
P.S. Garritan - you need to tune your boy choir, and maybe add ‘cathedral trebles’ and ‘countertenor’ to your library.
Ooh, can you give a link / instructions?
I like the authentic sound of Tenors that are slightly under the note! 
It goes by several names (Flobakks, Papelmedia, etc) and I believe it was part of the Korg M1 package. Originally sf2, but I converted in Plogue, though Aria may read it straight in from sf2.
Here’s an sf2 (free, register to download)
https://www.polyphone-soundfonts.com/en/files/21-vocals/222-korg-choir-ahhs
Demo here
I also found some choir WAVs but I have not checked them out – but am about to!