We are experiencing the same issue since 7/20/2023 after our scheduled sync got stuck "in progress" state. I followed the steps in the ArcGIS blog to troubleshoot, but the schedule has remained "blocked." The blog unfortunately does not include the method for how to unblock schedule. We also have the portal to AGOL collaboration (copy option). I also did the same thing (pause/resume) as well as deleting the schedule and re-setting the schedule to no avail (for unblocking). At least the manual sync is currently working. If someone was successful unblocking schedule, we would like to know the method, too.
In the past, when this "blocked" schedule happened, sometimes the "blocked" state went away by itself without doing anything. (It is not happening this time.) We also asked our GIS administrator to reboot the server in the past, and it worked to "unblock" schedule at that time, too. (This time our GIS admin suggests that we open a ticket with esri.)