figured it out!
I checked the querry phpmyadmin creates by using the button 'delete'. The automatically created querry was
DELETE FROM aabb WHERE 'Ref' = '101'
but it should be Ref not 'Ref'
when I made the querry manually
DELETE FROM aabb WHERE Ref = '101'
it works.
So it seems that phpmyadmin or my host has a bug by putting ' ' around the column name, when using the delete button instead of typing the querry manually.
Thanks anyway for your replies, at least I knew that I didn't do anything wrong and knew were to look for the mistake.