The non-return valve is incorrectly positioned in the dishwasher.
Remove the non-return valve. Proceed as follows:
Press the catch on the non-return valve inwards (1).
Tip the non-return valve inwards to release the cover (2).
Check the drain pump impeller by manually turning it.
Replace the non-return valve by placing it down vertically from above.
Make sure that the non-return valve is flush with the edge.