There are not currently any low-cost airline routes between Paris (Charles de Gaulle) and Cologne-Bonn with Germanwings.
Instead you could try browsing budget flights between Paris (Charles de Gaulle) and Cologne-Bonn with all airlines.
Another option would be to try browsing all low-cost airline routes to Cologne-Bonn or all budget routes from Paris (Charles de Gaulle).
In the meantime, this error has been logged.