As a Liebherr Engineer I have seen this problem before on not only Liebherr but various other makes.
The smell is always due to a spillage in the fridge allowing this to collect in the evaporation tray.
If it was milk or cream it will curdle with the heat from the compressor which is only designed to evaporate the defrost water.
On the K3150 the compressor has to be released from it's mounting pins and pulled out slightly to remove the drip tray. Some bending of the refrigeration pipework is required and should be done very carefully.
Or better still callout an approved Liebherr Engineer and get it remove proffesionly, for cleanig.