All containers and groups in the USM portlet are empty (no devices).
Someone deleted all devices through the device deletion function of the UMP USM portlet and failed to uncheck the "Prevent rediscovery" option from the Delete device dialogue. This caused the discovery_server to create an entry for every deleted device in the excluded_devices.csv file which prevented the discovery_server from ever discovering these devices again.
Deactivated the discovery_server probe and since all of the entries in the excluded_devices.csv file needed to be rediscovered, this file was deleted. On activation, the discovery_server probe was able to successfully rediscover these devices again.