Wanhao duplicator i3 print bed support warped
okay, after ferociously dismantling the whole bed I found out that the bed and the bed support are perfectly straight!
So there is something  warping the bed and support. De Y-axis bars are perfectly flat too. 

I found out that my table is not flat! This made the complete printer warped and even bent the print bed. put some cardboard under the frontleft foot and presto! As flat as it should be!