Logic Pro X Logic Pro X - Pops and Clicks

Can anyone tell me why I'm experiencing pops and clicks on playback when I don't have the same issue running an identical project in Reaper?

My audio interface is a Fractal Audio Axe-FX 2 running via USB but this project contains only MIDI software instruments, no audio.

Logic buffer size is set to 32 (although I have tried other settings).

None of my track levels are running into the red.

My machine is a 2007 iMac with 4GB of RAM.
 
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