Okay, got some replies from my distributor. The RAM is tested to work with 1.35V.
He's got some ideas though:
* The timings are DIFFERENT when running with 400MHz (DDR3-800) and often not in the datasheets. If we're running at that speed, we might have to adjust those. The timings can be found on the JEDEC Specification on page 170 and later (I'll send the document to you via Mail, as that's not public).
* Also, if we're running at low speed, the DLL can interfere with the RAM. This can be disabled via Mode-Register (might be something we can try).
* As it's dual rank, it could be the layout needs to be adjusted (as it has more load on the lines). However, then I guess it would run unstable at 1.4V as well, wouldn't it?
Anyways, that DLL and the timings are our best guess for now, I think.