I have a similar problem. We also have an external database which is connected with wordpress. Yesterday I sent a newsletter with your plugin for the first time – everythink worked fine. Afterwards I then removed the email addresses of some subscribers by hand. Now the links and attachments of all the sent mails are broken:
“Abonnent nicht gefunden
Text below only visibile to administrators
This tracking link contains a reference to a subscriber no more present”
I’m not sure if this has anything to do with the deleted email addresses. Any suggestion how I fix this? THANKS!