The E08 preheating error code appears if the set oven temperature is not reached within 50 minutes. If this code appears, the reason might be due to overheated thermostats, a defective heater or a defective relay; in which case a repair will need to be arranged.
However, if food has been put into the oven during the preheating time that may cause the error code, since the presence of food cools the oven and so it doesn't reach the right temperature. The error message can be removed by turning the oven off and turning it on again.
To prevent this error code from happening again, the oven should not be used during the preheating period. The preheating period ends just after the sound signal and the icon in the display have turned off. When the oven gives the signal, it's close to the set temperature and your food can be put in.