I assumed you would do that step by step not all in one go, sorry if that wasn't clear.
Question that need answering: Are both memory sticks working fine?
Stick A works fine at 2133Mhz in far right grey slot ?
Stick A works fine at XMP 2400Mhz in far right grey slot ?
Stick B works fine at 2133Mhz in far right grey slot ?
Stick B works fine at XMP 2400Mhz in far right grey slot ?
Stick A works fine at 2133Mhz in far left grey slot ?
Stick A works fine at XMP 2400Mhz in far left grey slot ?
Stick B works fine at 2133Mhz in far left grey slot ?
Stick B works fine at XMP 2400Mhz in far left grey slot ?
Stick A + B both grey slots at 2133Mhz = ?
Stick A + B both grey slots at XMP 2400Mhz = We know crashes
Fill in the blanks and you will have some answers.
Assuming all stable at 2133Mhz, CPU is having issues running the 2400Mhz.
What can happen over time is the CPU changes slightly, degenerates so what once worked no longer does.
If XMP 2400Mhz only crashes in one of the Grey slots, points to CPU memory controller, related to the above.
How to resolve?
Hard to say as if your CPU is the cause then its possible you will never be stable over 2133Mhz. Some kits provide higher memory timings, those numbers you see 2400Mhz 16-16-16-X.
Do you have access to any testing parts, friends RAM, CPU?