This may not be relevant to your problem, but I have a UR424 and experience occasional, but serious clicks in recorded files. It’s a stereo interface so I would only record 2 tracks at a time, I use and elderly MacBook Pro running 10.11. As you say, it looks like a clock slip.
I was not able to resolve the problem. I posted on the Sternberg hardware forum and searched, there are other posts there with similar issues IIRC but no solutions. I suspect a driver issue.
I have now dumped the interface and use the little Audient, which is faultless - everything else in the system the same, it confirms to me the Steinberg interface has a problem. I won’t be buying Sternberg hardware again.