Ok just wanted to check that's all as i know what the fault will be here as ive seen it loads of times.
The drain hole at the back will be blocked so what happens when it goes into defrost mode the water cannot drain out so it builds up and up and causes a big block of ice behind the rear panel.
Even though its frost free it does frost over then a heater comes on and melts the ice that's how it keeps it a frost free. But sometimes if you can get a fault in there like either a faulty defrost heater or a blocked drain it causes the ice in there to build up and the start causing trouble in there.
As you are looking for a switch for the ventilator as per the manual there isn't a switch.
What the manual means is when the control board tells the appliance to go into defrost mode the control board turns the ventilator off itself its not something you need to do as this is a full electronic controlled appliance it does it itself so theres no switch in there you need to mess with.
As to the fault you have theres 2 things you need to do. Either empty it and turn it off and leave the freezer door open for 24 hours and allow it to defrost itself.
Or empty the appliance and remove the rear panel inside the freezer and from there use a hairdryer to melt the ice away.
Once that's all clear you will see the drain hole in the bottom of the drain gutter. Poke something in there and pore water in there and make sure it drains out ok.
Once that's all clear refit the rear panel and turn the appliance back and this will cure the fault from there.
If you need anymore info just get back to me.