The answer to 'ductwork freezing up' is that it is typically caused by a lack of airflow or a refrigerant leak. Symptoms include ice buildup on the ducts, reduced airflow, and poor heating/cooling performance. Solutions can include cleaning or replacing air filters, fixing refrigerant leaks, and improving airflow through the system.
If the ductwork is freezing up, it's best to have a professional HVAC technician diagnose and repair the issue. Attempting DIY repairs on an HVAC system can be dangerous and may void the warranty. A qualified technician can properly identify the root cause and make the necessary repairs to get your system back up and running safely.