Found the problem. It turns out that somehow my Designer account was not linked to the correct Controller MAC address. I'm running Controller on a Rpi and the Controller MAC address was different from the one linked in the Designer. At first I was unable to add update the Controller MAC in Designer, until both the wired and wireless MAC addresses from the Rpi showed on the Controller sync page. I tried added both separated by a comma and Designed accepted the new Controller MAC addresses; once I synced the design again everything worked.
No idea how the MAC address could have become un-linked or changed either on Designer or on the Rpi itself. But it's fixed now.