White Screen on Power-Up

Hi,

I've got a problem with my iMac.

When I'm powering it up, im hearing the chime and than immediately get a totally white-screen.

So this occured since a few months, and today I powered it up again, seeing the Apple-Logo and then the Login-Screen! I was in a super-happy state as this happend.

But when I rebooted it, it still showed the Apple-Logo (-> Display & EFI working), but it stopped spinng the loading-cursor...

It did that about 10 times, until I did a NVRAM-Reset, from what moment on, it again displayed nothing but a white screen on power-up.

So, i did an NVRAM- and SMC-Reset a few times and than taking it apart, removed the HDD and the ODD (including cable from Logic-Board) as especially the ODD cable should be a weak-point causing this issue.

Still, nothing but white.

So, I need your advise... Does someone of you had kinda the same issue, or has some helpful tips?

Thanks very much in advance.

Arne