This error message is shown if you attempt to open a Pro Tools session that is at an entirely different sample rate to your current one.
e.g. You are working in a 48 kHz session and try to open a 96 kHz session or vice-versa.
Essentially, it happens if you attempt to move between fs, 2fs and 4fs sessions.
This is expected Core Audio Driver behaviour.
To resolve this and be able to open the session, simply change the sample rate in the SSL 18 Mixer in SSL 360 or in AudioMIDI setup before you open the session that’s at a different rate.
Extra Note – you may find that if you have attempted to open a different session and experienced the above error message, you are then prevented from opening different sessions at the same sample rate. If this happens, set the correct sample rate in the SSL 18 Mixer or AudioMIDI and then load the Pro Tools session.