Hmm, You may want to give it a try with just 1 DIMM installed. Then update the BIOS (and the rest of the code stack)
BIOS contains the Memory Reference Code and some DIMMS need newer code to correctly be configured.
thank you for you answer. We checked it with one DIMM - but its also beeping with just one module installed. At the moment we are trying to find some older DIMM modules.