F18 is drain fault.
As you have already checked the drain hose that connects to house drain and pump filter is clean, please try this:
You will have to unplug the appliance and pull it out.
Then remove the top off the appliance and locate the pressure switch with thin hose attached to it.
Now, remove this hose and blow through it, make sure it is not clogged.
Then with the top off the appliance, locate the hoses going into dispenser and clean them completely.
Now lay the machine against the wall or let someone hold it for you and access the black sump hose that connects drain pump to the drum.
Remove this hose from pump and make sure it is not clogged, clean it completely.
After you have cleaned everything then reconnect and plug it back, problem should be solved.
Please let me know how it goes.