E20, E21, and C2 on AEG belong to the drainage family; documented models may also emit two beeps or flashes.
Start with installation and outlet
The manufacturer documentation includes E22, E23, and E24 among the related alerts. Keep the subcode: grouping them together to check drainage does not let you assign a faulty part to each one.
Check the hose route, how far it extends into the outlet, and the height relative to the manual diagram. If it drains into a sink, see whether there is a blockage or backflow in the household drain. Check for suds if they accompany the retained water.
Accessible filter with controlled draining
If your PNC has a user filter, stop and unplug the appliance safely, let the water cool, and prepare a container. Drain it using the indicated method before removing the cap completely. Clean only accessible debris and reinstall the seal tightly.
The guide to cleaning the drain filter expands on the preparation. If there is no intended access or the water is still retained after checking the route, request service. A hum does not prove that the flow is correct or confirm a faulty pump.
Do not force the door open to remove clothes while the drum still contains an unsafe level of water.