This could be due to various reasons:
You have potentially placed too many and/or too warm products in the appliance.
The lid is not closed properly or the seal is broken.
The appliance is inadequately ventilated. There should be a minimum spacing of 100 mm from all air vents.
The ambient temperature is too high (above 42°C).
The temperature has been set incorrectly.
If these measures do not eradicate the problem, then please contact Truma Service, who will be more than happy to help you.