Reporting similar symptoms on a long drive, occurring about an hour into a three hour drive. Noting the weather was clear and hot. Same icons, same message.
Shutdown and restart did not clear the message. Wanted to disconnect car battery and reset car computer, but didn't have any tools onboard.
Opted to continue driving without Smart Cruise, error icons present. After maybe half an hour the error cleared itself - error icons went away. Checked the front badge (radar cover) for excessive bug splatter, but it was minimal. Smart cruise resumed normal operation thereafter.
Google Search returned the following:
Smart Cruise Control on an i30 might not meet conditions for a number of reasons, including:
Weather
Bad weather, like rain, can reduce visibility and prevent Smart Cruise Control from working properly.
Windshield
Moisture on the inside or outside of the windshield can interfere with Smart Cruise Control.
Sensors
Blocked or dirty radar sensors or cameras can prevent Smart Cruise Control from working.
Electromagnetic waves
Strong electromagnetic waves can interfere with Smart Cruise Control.
Faulty control module
A faulty control module can prevent the cruise control from engaging or maintaining a consistent speed.
Electrical issues
Issues with the car's electrical components, like a loose wire, damaged alternator, or electrical short circuit, can prevent the cruise control from working.
Considering the smart cruise was working flawlessly up until that point, none of the above seemed to be the cause.
However, the google search did present this option:
"If Smart Cruise Control is unavailable, you can switch back to conventional cruise control. To do this, press and hold the adaptive cruise control button."
Ultimately, not the solution, but better than no cruise.
Looking forward to a definitive answer on this one.
Cheers.