We have had our AEG Lavamat Oko 86720 for over 8 years. We too got the C9 Error Code.
I managed to reset it by Holding the Start Button whilst turning the dial through 360 (from Off-2-Off). I did this twice and then the second time normal operation was resumed.
Not sure how long this will keep the C9 error code away, but at least it allowed us to continue using the machine.
Simple Simon, January 2009