Why does the water drain only when it’s empty?

Why does the water drain only when it’s empty?

I have a 4 year old front load washing machine. If I run it empty (with no clothes), it goes through the entire cycle and drain perfectly with no issues. If I add clothing, it stops toward the end of the cycle, clothes are still soaking wet and there is water that has not drained. The machine beeps with an error message E21. I have had 3 people look at it. Drain is not clogged. Can’t figure out why it runs fine with no clothes but not with clothes.