One of my colleagues has just been through this rigmarole with the review process.

Apple kept rejecting it as it would crash when logging in. We didn't get any crash reports and could not replicate any issue ourselves. Several submissions later, the decision was made to put in a whole bunch of analytics and tracing, we get the logs back and... The "crash" was a pretty clear error message because they weren't following the simple 2 step login instructions provided to the reviewer.