Hi,
If the Adblue level was correct then this is almost certainly due to a NOX Sensor failure.
If the warning message haunt changed yet expect it to change to something like Adblue system failure engine start not possible in and it will countdown from 500 miles,
This is very common apparently, mine went on my E350CDI 65 plate, mine were both replaced FOC by MB.
Has youre car been recalled for an Engine ECU update? As there is reason to think that this can cause these sensors to fail prematurely, hence why a lot of owners seemed to be getting them replaced under goodwill/warranty.