System overload. The audio engine was not able to process all of the data in time.

"System overload. The audio engine was not able to process all of the data in time."

I keep on getting this error message, and my song stops playing.

I have only six active tracks in my song.

The problem occurs even after restarting the Mac and quitting major programs such as Photoshop, Indesign and Dreamweaver.

Activity Monitor > Memory says:

Physical memory: 16 GB.

Memory used: 5.85 GB.

Cached files: 4.18 GB.

Swap used: 0 GB.

I also tried "Show Track Lock" but that also didn't resolve the problem.

I'm using a mid-2015 Imac. My songs are on the Imac and there are almost two terabytes of free space on the Imac. My system is Yosemite El Captain 10.11.3 (the latest). My Garageband is 10.1 (also the latest).

Tony