Missing input parameters error on opt-out form
The error seems like it might be happening because one of the jid, qid, h params  are not present. 

As a first step - I'd make sure that the opt out was working properly with a test contact (send them an email and opt out via the link and confirm it's working).

Then if that is working I'd check your server logs - it's possibly something hitting the url without the correct params (some sort of automated crawl etc).

If you wanted to you could try editing the file /home/user/wordpress/wp-content/plugins/civicrm/civicrm/CRM/Utils/Request.php  on line 31 and output further information about the request. Definitely back up that file first!