Diagnosis
That's the 'EF' error code, which stands for 'Electronic Failure' or 'Electronic Fault'. It's usually triggered by a glitch in the communication between the main control board and the humidistat sensor.
How to Fix It
First, unplug the unit from the wall for a full 5 minutes. This is a hard reset that clears the control board's memory. After 5 minutes, plug it back in. The 'EF' code should be gone, and you can set your desired humidity level again. This fixes it 90% of the time.
If That Didn't Work
If 'EF' comes back immediately or after a short run, the humidistat sensor itself might be faulty or have a loose connection. On this model, the sensor is built into the main control board assembly. You'd need to open the case to check connections, but the likely permanent fix is replacing the main control board.
Heads Up
Don't just flip the power switch off and on on the unit itself—that's not a full reset. You have to physically unplug it from the outlet for the full time to drain the capacitors on the board.