We are using sitecore 8.2 update 1 with EXM. Mailing campaigns isn't a problem but then when the user clicks on the unsubscribe link they don't see any success message. It's just empty page. Sometimes we see '404' page and sometimes its empty.

Server configuration is 1 CM + 1 CD server. On CD, AnalyticsClusetername is set to one of the site's host name and AnalyticsHostname is empty.

Also, the FinalConfirmation page is also set on the exm manager root,

In templates (both EXM OOB and custom templates) the link to unsubscribe is set as /sitecore/Unsubscribe.aspx But still the unsubscription doesn't work. No message and the contact is not added to global opt-out list.

Logs show the below message. Apart from it, there are no significant errors:

The query string received for this event was not marked as valid. The event will not be handled.