What my be the cause of controller icing?

Question

Question Quiz BPL Gas - Balloon Pilot License - Operational Procedures

QuizVDS · Accepted Answer

Liquid gas has entered the controller.

QuizVDS · Answer

A cloud has been entered at temperature between +5 und -10°C.

QuizVDS · Answer

Temperature in the controller is between -1 °C and -5 °C.

QuizVDS · Answer

Relative huimidity is higher than 50%.

Id 392 - BPL Gas - What my be the cause of controller icing?