Without the information requested in my previous response, I can only give you a general answer however, I hope this helps.
This indicates temperature issue and/or an air flow problem due to heavy ice build-up inside the rear wall of the appliance.
Please try this to solve the issue:
Leave it unplugged and empty with door open for around 5-24 hours.
Then plug it back (expect some water on the floor) and wait for another 3-8 hours so it gets back to optimum temperature.
The problem should be resolved.
However, if the issue appears again then the problem is with defrost operation.
You will have to test/replace the temperature sensor and/or defrost heater element to resolve the issue.
Alternatively, you may call Neff at 03448 928 989 to book repair.
If there is anything else I can help you with, please let me know.