This is most often caused by a plugin so follow the steps below to find out which audio plugin or virtual instrument is causing it.
1. When it happens it usually happens in the same time position in the song, so reset the audio device by going to Options/Preferences > Audio Setup > Audio Device, and chose another audio device. Let it "take", then switch back to the original audio device. This resets the original audio device.
2. Play the song and take note of where on the timeline the 666 error occurs. Repeat step one, reset the audio engine and play it again. If it happens in the same place again, loop that range, maybe 8 bars before where it happened.
3. Play that looped section and start disabling tracks one by one until it no longer goes 666. When it stops, the offending plugin or instrument causing the 666 error is on that track. If it's an audio track, rinse and repeat while disabling individual plugins on that track until you find the one causing the issue.
4. Once you find the offending plugin or instrument you can try another different format (VST2 instead of VST3, etc) and you should also report it to the plugin vendors tech support. If it only happens with that plugin when using a particular audio device and driver, say Focusrite, and it doesn't happen otherwise, make sure that the audio device driver is fully up to date and if not update the driver and test it again.