I also have a kenwood head unit in my car. I have got Protect error many time after getting my car washed....
Although my head unit is not the one u mentioned, its some other model.
Solution
The solution i found is that there is a very tiny button (Reset i think) at the right bottom corner of the head unit like the one found on WIFI router to reset it which can be pressed using a Pin. I pressed it using a common pin and the error was solved !
Now every time i get the protect error i press the button . Hope u also have one on ur head unit 