Ok E21 is a wash motor fault and E15 as you now is a water leak in the base.
Now what I find a lot on these is the wash motor leaks and cause water in the base to create the E15 error code. So tipping it backwards will clear the water and reset it.
But a lot of the times on these its the wash motor that leaks and in the end it causes the motor to jam up and will cause the E21 error code.
What you need to do is remove the top and the right hand side panel.
There at the bottom you will see the wash motor. Have a look at this and see if theres any signs of a water leak. Also check to see if its jammed up as well. You will see a metal impeller fan at the back inside the motor. Try and turn it by hand and see if it turns. If its very stiff or not turning then the motor is faulty and will have been causeing the water leak you have been having in the past.
So check it out and see what you find. But the E21 error code is a fault on the wash motor.
Hi, Right panel removed, I see no wash motor.. a white plastic box... you sure you are thinking of a Bosch silver edition 40A08GB?
Ok the motor is behind that plastic box so remove this and you will see it.