[Lemoncel]

I get this too, usually shortly after launching PT. I have the same setup as you - iMac-El Capitan-Saffire Pro 24.
What works for me is to open Playback Engine settings (under the "Setup" tab) and change the H/W buffer size. That will usually do the trick and I don't encounter the error again. If I have a lot of VI's in the session, I will typically get the -9173 error, and the same process will get rid of it.

Hope this helps.