Hello Stefandg, thanks for joining the Intel community.
It is important to mention that this system combination has not been validated by Intel. The following link will show you the list of processor compatibles with the DX58SO motherboard. http://processormatch.intel.com/Processors/CompatibleProcessors?componentName=DX58SO
Bear in mind that the Xeon 5650 processor is not unlocked; therefore it may be not allowing you to change the memory configuration. In addition, if you are setting a very high XMP on the system, it may not be able to handle it.