Definitely not right, in my limited knowledge of the HVAC system, it may be that the refrigerant used is incorrect, or as you have said the sensor malfunction.
The refrigerant needs an extremely low temp to freeze, which is not an easy thing, also the reason why it's kept under pressure to help mitigate this too. (PV=nRT), so could also be that the line has some air or other liquid in it that is freezing over. Best to see a vehicle AC specialist. Dealer may over charge you.
Are all the line for the AC frozen or just this one section, I see it's on the hose with one of the H/L pressure valves.