Water backing into dishwasher WITH new check valve and loop in place?
Fill kitchen sink with water and drain it while observing high drain loop if it is translucent.  Or disconnect drain hose from Dw and drain sink . If no water travels to the dishwasher the issue is with dw not getting rid of waste water correctly. 


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If water is traveling from sink to dish washer there may be a change in how your sink is draining like a partial blockage in drain line or an obstruction in plumbing vent.

Edit
Since you commented that you tried the experiment above but saw no water in dishwasher and sink drains well the only other thing I can think of is perhaps it is filling with fresh water. A faulty inlet valve could cause this. Even fresh water will get gross if it sets in the bottom with the traces of bacteria laden gunk in the strainer etc... 
Try turning supply off for a week to test this theory.

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