In our Massmailing campaign (Odoo 12) it is necessary that contacts have the possibility to unsuscribe.

Clicking on the unsuscribe button results in Error 403

Forbidden

You don't have the permission to access the requested resource. It is either read-protected or not readable by the server.

In the Odoo Server Log, I can trace the request, but I cannot see any errors, also not in syslog, or database logs.

Are there any ideas?