Opened a case with Tableau support on this and this is a known issue with versions 2018.2 and higher
A workaround is to delete all the current subscriptions for the site and then enable self service schedules.
that explains why I had no issues in our DEV environment - because there were no subscriptions defined yet.
When you did this, what impact did it have on your users? Did you/they have to recreate a significant number of subscriptions, or was it pretty straightforward to recover from?