Cloud Build for iOS fails with: panic: runtime error: invalid memory address or nil pointer derefere

I'm in the process of setting up Cloud Builds, Android and Windows builds work fine after some initial setup, but my last four iOS builds have failed half way with this error:

This thread seems to indicate it's related to performance reporting, so I've tried disabling that. No luck.
My project is on Unity 5.6.2f1, which seems to be a newer version than the one referenced in the thread, so I imagine that fix should have ended up in there too?

I have also tried both XCode 9 and 8.3, seems to make no difference.

I have had builds succeed, so it's not permanently broken, but it's unclear to me if it's something I'm doing wrong or just a hiccup on the server. Having to wait ~45 minutes between attempts isn't helping either.