DueSlot“Publish was unsuccessful” in Planner
If Planner shows “Publish was unsuccessful” or will no longer give you an iCalendar URL, that matches Microsoft's retirement of the Planner Outlook calendar feed (MC1193421).
There is no official feed to repair. DueSlot is an alternative: it writes events to a calendar in your own mailbox using Microsoft Graph, instead of publishing a public or mailbox iCalendar URL.
DueSlot does not offer a hosted iCalendar feed in this version (an optional feed may come later, off by default).