I believe I have found the cause of these errors. Microsoft Outlook maintains its own email domain reputation system called: Outlook.com Smart Network Data Service. It is completely separate from all the other spam lists, so many people do not realise that their email domains deliverability is affected by this. I discovered my mail domain IP was listed as Not qualified for mitigation, which translates as “treat as junk”. I was pretty surprised to find this out and quickly got my IP off the list (all IPs have a history of course).
A side effect of getting my IP clean in the eyes of MS Outlook has meant I no longer see the mautic Invalid clickthrough value error. So it would appear that it is the Outlook Smart Network Data Service that is the root cause of this error.
You can request that your domain IP is removed here: Smart Network Data Services