I just upgraded my store to 4.60. I'm seeing this in the error log. I've double checked that the NewsLetterSubscription table does not have a blank email.
MailChimp error. Object reference not set to an instance of an object.
System.NullReferenceException: Object reference not set to an instance of an object.
at MailChimp.Net.Core.BaseLogic.Hash(String emailAddressOrHash)
at Nop.Plugin.Misc.MailChimp.Services.MailChimpManager.GetDeleteSubscriptionsOperationsAsync()
at Nop.Plugin.Misc.MailChimp.Services.MailChimpManager.GetSubscriptionsOperationsAsync()
at Nop.Plugin.Misc.MailChimp.Services.MailChimpManager.<>c__DisplayClass76_0.<<SynchronizeAsync>b__0>d.MoveNext()
--- End of stack trace from previous location ---
at Nop.Plugin.Misc.MailChimp.Services.MailChimpManager.HandleRequestAsync[T](Func`1 request)