You've answered your own question. E20 is a draining failure.
Blockage in the sump hose, pump, drain hose or faulty pump.
Clear the blockage or replace the pump if faulty and you'll clear the fault.
Use calgon in future, it's probably blocked with limescale. The common error codes are listed in the manual if you've still got it. E20 is the most common, and as said is a drain filter blockage. If you've lost the manual, you can get it off the interweb.
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