Hi,

1. Likely causes (ranked)

   • Memory rank/firmware/PCB trace differences between manufacturing revisions (the CWEC0 vs CWEBY suffixes). Even with same main P/N, Samsung/ODM changes (SPD firmware, minor timing/PCB layout, ECC/thermal/resistor packing) can make modules incompatible when mixed.
   • SPD/EEPROM mismatch causing the memory controller to reject a mixed configuration.
   • Subtle voltage/timing differences (one set running at a different default/tighter timing) causing training failure when mixed.
   • Broken or counterfeit modules (less likely since new set boots alone, but could be a subtle compatibility variant).
   • CPU memory controller sensitivity — some Skylake/Skylake-SP controllers are picky about mixing ranks/manufacturers even if P/N matches.
   • Slot/trace or seating issue revealed only with mixed population (less likely if pure sets work).

2. Quick tests to narrow down

   • Boot with only one module per CPU: try one old in CPU1 and one new in CPU2 (and vice versa). You said 1 per CPU fails — confirm combinations (old in slot A1, new in slot A1, old in A1 + new in B1, new A1 + old B1). Document which specific slot combos fail or pass.
   • Swap CPUs (if you have two) to rule out one CPU’s IMC being sensitive to a module revision.
   • Update/verify microcode + BIOS again (you said updated; confirm exact BIOS/CPLD/iDRAC versions and post codes).
   • Check iDRAC/System event logs for memory training errors or SPD read errors — capture the exact error message or POST code.
   • Use one old + one new in same channel (A1 and A2) vs across channels to see if channel mixing matters.
   • Try lowering memory speed in BIOS (set to 2666 or manual timings) and test mixed population — if training succeeds at lower speed, it’s a timing/gear issue.
   • Run Memtest/Diagnostics with homogeneous sets (already pass) then with mixed sets if you can get it to POST with conservative settings.

3. Settings to try

   • Force memory speed to a lower JEDEC rate (e.g., 2666) and set conservative timings.
   • Disable any XMP/overclock/gear settings and set voltages to nominal 1.2V (or the JEDEC voltage shown by SPD).
   • If BIOS has “memory training” options, enable/disable extended training or clear NVRAM/CMOS.

4. If tests point to SPD/firmware mismatch or persistent incompatibility

   • Return/exchange the new modules for the exact same lot/revision as the originals (ask reseller to match the revision suffix).
   • Ask reseller/Dell for an RMA/exchange and supply the module FRU, part number, serial numbers and the server service tag; reference that mixing same P/N but different revision prevents boot — request testing on a R640 or validated compatibility confirmation.