Found a solution. Apparently the logon.aspx file has been changed after CU4 much larger.
We got the Form SSO Profile working on the later CU just by changing the Response Size value from the original value 10240 to 60000. The actual limit is between 50000 – 50500 but we put the value to 60000 just to be safe.
BTW… The SSO seems to work just fine without the pback rewrites. We didnt find any problems if we disabled the rewrite. Is this still needed?