There are two pairs of electric sensors on the left and right that try to detect a high water level.
Look in during self-cleaning to see if there is foam. Foam can trigger a false alarm. It did for me.

If so: Empty the fresh water tank, fill it with water only, start the self-cleaning, if there is an error message, press continue, then start the self-cleaning again until about 4 self-cleaning cycles have been completed.

Afterwards, use a cleaning agent that does not/barely forms foam. Maybe dissolved dishwashing detergent or all-purpose cleaner? I don't have any really good advice. Or use much less of the existing one.

If everything has already been cleaned, only sending it in under warranty can help so that the item can be checked. May be a sensor fault that would be impossible to determine remotely.