Scheduled Reminders problem in V 4.7.16
As a quick-fix, remove the {action.optOutUrl} token.

To pursue a more general fix, there should be an issue in JIRA. One needs to determine whether:


The {action.optOutUrl} worked in a meaningful way in previous releases. (In this case, we need to define what that was and figure a patch to restore it.)
Or the {action.optOutUrl} never worked -- and merely became symptomatic in 4.7.16. (In this case, we need to manage the symptom better -- e.g. present a alert to encourage admins to fix their scheduled reminders.)


(If you don't have a JIRA account but can describe the expected behavior, then post a comment here, and I'll try to file it.)