Hi gulkamit;
Welcome to the forum.
It seems that you did most of the trouble shooting for this error. You should also make sure that there is nothing blocking that connection ie: loose particles, dust. Try using a piece of steel wool or super fine sandpaper to clean the ends of your sensor cable works well.
Also; while the generator is on and the "E1" error is displayed and beeping, check the connection by wiggling one side of the connection up and down and left to right, and then the other side. If the “E1” message temporally goes away while you were wiggling one side, you need a new sensor connector. If you have a bad sensor contact Bradley.
The -6°C would still be in the operating range of the generator, unless the generator was left overnight in an unheated area and the generator got much colder. Usually you can just bring the generator indoors to warm up a bit. When you did your test run a few days ago, do you recall if you brought the smoker up higher than 280°F (138°C)?