Our site is producing fatal "Missing input parameters" errors from the mailing opt-out form. This happens several times every day at random times. I have looked at a few other SE posts with similar errors, but those issues seem unrelated. The log output is lengthy so I dumped it here. If anyone could decipher it and let me know how to debug this, I'd appreciate it. Thanks.
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!