Password Sync fails with "Retry queue limit has exceeded configured limit"

I have set up Azure AD Sync for our two domains (staff and student). Our staff domain passwords sync ok and work fine, but when Directory Synchronization gets up to the student domain, this is logged in the event log and password sync stops and logs Event ID 611.

Password synchronization failed for domain: ourdomain.wa.edu.au. Details:
Microsoft.Online.PasswordSynchronization.SynchronizationManagerException: Retry queue has exceeded configured limit: 5000.
   at Microsoft.Online.PasswordSynchronization.RetryQueue.Enqueue(IEnumerable`1 items, Int32 maxRetryQueueSize)
   at Microsoft.Online.PasswordSynchronization.SynchronizationExecutionContext.UpdateRetryResults(ISynchronizationTask task, IEnumerable`1 retryItems)
   at Microsoft.Online.PasswordSynchronization.SynchronizationExecutionContext.ProcessTaskResults(ISynchronizationTask task, SynchronizationTaskResultType resultType, SynchronizationTaskResult results, IEnumerable`1 retryItems)
   at Microsoft.Online.PasswordSynchronization.SynchronizationExecutionContext.SynchronizeDomain()
   at Microsoft.Online.PasswordSynchronization.SynchronizationManager.SynchronizeDomain(SynchronizationExecutionContext syncExecutionContext)
.

ourdomain.wa.edu.au

I have set the ErrorLimit registry entry, but it does fix the problem. Any ideas?