This has absolutely nothing to do with locked vs. unlocked processor.
When this happens, it is usually because the memory cannot be reliably accessed at this higher speed. The problem with the process for tracking down what is responsible is that it could be the fault of so many components. This includes the motherboard's BIOS, the motherboard itself, the DIMMs or the processor. I have listed these in usually-responsible order, going from most-likely responsible for problems down to least-likely.
I would point out that this processor is only validated by Intel to run at up to 2666MHz. Any speed faster than this is, as they say, a whole new ball game and not guaranteed to work. This would apply equally to locked vs. unlocked processors. XMP is a separate form of overclocking and is just as likely to succeed (or not succeed) with an unlocked processor vs. a locked processor.
The issue here is (electrical) noise. Every component generates some noise. As they age, components can (and usually do) generate more noise. If the overall amount noise that is being picked on the memory buses (which can act like antennas) reaches a certain threshold, a receiver will lose the ability to reliably differentiate data from noise. This could occur at a transceiver in a DIMM or this could occur at a transceiver in the processor (or both). The higher the frequency of the memory buses, the more susceptible to noise the components are. The motherboard will have circuitry - including things like termination resistors to avoid signal reflection on the bus - to help (attempt to) dampen noise. The higher the quality of the motherboard, the better this circuitry will be. When differentiation breaks down, the memory buses can lock up. When this happens, the system can hang. Usually, a Watchdog Timer resets the system when these hangs occur for a certain length of time.
The BIOS is responsible for initializing both the DIMMs' and the processor's memory controllers to run at a specific frequency. Usually, if a frequency cannot be maintained, a breakdown will occur quickly enough that the BIOS has an opportunity to try running at a alternate (lower XMP or base) frequency - and, in fact, this may occur during BIOS POST without you even knowing it is happening (unless the BIOS stops to tell you). In some cases, however, the lockup that results is such that a full reset of the processor, DIMMs and motherboard (memory buses) is necessary to get it going again.
I would also point out that most high clock rate DIMMs recommend that you use only one DIMM per memory channel. When you use two DIMMs in one memory channel, you are raising the likelihood of an issue at higher frequencies.
Hope that explains it,