Just a quick update:
notaz sent me a U-Boot version that runs the 4GB RAM with DLL off and 1.5V.
That seems to work fine. I was able to boot multiple times without any issues (so no heat or similar).
The unit I tested is now running for 2.5h. It's only idling around so far, but I'll try some more stuff like memtester, etc. I also need to test this with the other boards, and I also need to test 1.5V only (with DLL enabled).
Just the DLL Off version with 1.35V didn't work for me.
So yes, raising the voltage seems to do the trick. As Juliano simulated, we have a pretty high voltage drop (from 1.5V to 1.1V on some balls), and that would certainly explain that.
It really looks like our timings are fine and the noise / voltage drop is the main issue.
So what can we do here?
1. Try to improve the board design as much as possible. It seems we're not THAT far away from getting it to work under normal conditions, so some small changes (additional vias, some capacitors) could be enough.
2. Try to find the minimum voltage setting with DLL off that works with all 4GB RAM PCBs. If 1.4V would work reliable, that would at least be a workaround for the first batch. We should still try to improve the PCB though.