Hello Marcus,
I guess you are not alone: https://ticket.coreboot.org/issues/121 Looks like the same issue, on a T520 with Ivy Bridge CPU.
According to some reports, the combination was working before. Maybe it's a regression. The combination is probably not often tested.
Also, as you speak of the flash image as if it were a single entity: It's not. There are multiple firmwares and configuration blobs that share the flash chip. You shouldn't flash all of them at once. It can confuse the Management Engine (ME) and cause random errors. Hard to trace the issue down then, if you don't know which part of the flash (or the flashing process itself) causes it. You can check your coreboot log for ME errors.
Speaking of the ME, it might be possible that you need a version that supports Ivy Bridge. Just a random guess, I don't have any experience with the ME firmware shipped with Sandy Bridge.
One last thought, you should make sure that the microcode updates are applied.
Hope that helps, Nico
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Nico Huber