Burden of proof of establishing a lack of conformity
The main obligation of the seller is to deliver goods which are in conformity with the contract. This means quality standard, and it is also applicable to services. You can read more about conformity here.
Generally, the burden of proof about a lack of conformity falls on the consumer. The consumer must also establish that the lack of conformity existed at the time the goods were delivered.
However, in the Consumer Sales Directive, there is an article that puts the burden of proof on the seller: if the lack of conformity becomes apparent within 6 months of delivery, it will be presumed to have existed at the time of delivery. Only if this presumption is incompatible with the nature of the goods or the nature of the lack of conformity, it will not be applied.