Hopefully someone can help me understand something:
 
I have installed UVI Workstation for the very first time. I have a Focusrite 18I20 interface, but when I set my Audio/Midi preferences within UVI, I get a crash every time when selecting Buffers = 512.
 
The error is IRQL_GT_Zero_At_System_Service, and I get a hard OS crash.
 
This happens every single time.  If I set the buffers to 128, I have no issues.  I have no issues whatsoever with any other plugin - Aruturia, Play, Cakewalk, Synthmaster, etc. with higher buffer settings.
 
Does this make sense to anyone?

First, I'd make sure you are running the current version
of UVI, then make sure your are running the current drivers
of your Interface...

Current version of UVI is 2.6.9

I'm running current of both.

Is it crashing as the standalone UVI or as a VST plugin within Sonar?

Standalone.  I haven't tried yet in Sonar.

What version of Windows are you running?

I have no issues on Win 10

Win 10 1607.
 
UVI is of course blaming it on Focusrite, despite I have no issues whatsoever anywhere else.

I have UVI Workstation 2.6.9.x64 with "True Cowbells" loaded on Windows 10 1607, and switching the buffer size from the UVI UI sets my M-Audio ASIO drivers to any size supported.  No problems here.

abacab
I have UVI Workstation 2.6.9.x64 with "True Cowbells" loaded on Windows 10 1607, and switching the buffer size from the UVI UI sets my M-Audio ASIO drivers to any size supported.  No problems here.

Same for me with Tascam ASIO driver.

husker
Win 10 1607.
 
UVI is of course blaming it on Focusrite, despite I have no issues whatsoever anywhere else.

Ok...try rolling back to the previous version of the Focusrite 
driver and see if that resolves the problem.  I use a Focusrite
Solo and have no issues whatsoever...