A firmware update for Samsung SmartThings Hub 2018 and 2015 models was released to customers on Monday, December 9th between 11:30am EST and 2:30pm EST. This release version 28.12 is a hotfix for version 28.11 that includes a fix for device pairing for certain Z-Wave devices.
Your Hub briefly went offline as it rebooted and applied the update; most users will have experienced less than one minute of downtime. If you are still seeing issues with Z-Waves device paired between December 2nd and December 9th, please remove and reconnect the device. Visit https://community.smartthings.com/t/legacy-z-wave-devices-and-hub-firmware-28-11/179732
for more information.