Hello AndreasComit,   I is possible there is a bad DIMM but try this first: Try updating BIOS and test the 64G DIMMs   Dell Server PowerEdge BIOS R640  Version 2.19.1 [URL]   Populate A1 and B1 only with a 64G DIMM, test results Remove those DIMMs and put the other two 64G DIMM in A1 and B1 and test Add the other two 64G DIMM to A2 and B2 and check results   If you can try running the built in diagnostics Boot to  F11 on Dell Splash screen, selecting  Boot Manager -> System Utilities -> Launch Dell Diagnostics.  Note any messages and continue testing.
The server is working fine with the current configuration (24x32gb sticks) but we tried replacing 4 of the sticks with 64gb ones and the server would get stuck at configuring memory screen We then tried booting the server with A1 and B1 populated ONLY with our new 64gb sticks and it booted up just fine.  After that we went by the rule that states larger capacity sticks need to be installed first and both CPU’s need to be balanced. We tried A1, A2 and B1, B2 and it would still not boot.  Could this be a faulty stick?  The new sticks are SKhynix  (HMAA8GR7MJR4N-WM) Please advice if we are doing something wrong or if there’s something in the bios memory settings we can change so we can make it work.  Model    PowerEdge R640 Operating System             DellEMC-VMware ESXi Operating System Version             7.0 Update 2 Build-17630552 (A00) BIOS Version       2.18.1 iDRAC Firmware Version 6.10.80.00   CPU 1    Intel(R) Xeon(R) Gold 5218R CPU @ 2.10GHz CPU 2    Intel(R) Xeon(R) Gold 5218R CPU @ 2.10GHz   Current memory configuration Installed Capacity             768 GB Slots Used           24 Error Correction                Multi-bit ECC   All sticks are now 32GB   A1 M393A4K40DB3-CWE - Samsung/Dell                  A2 HMA84GR7DJR4N-XN - SK hynix A3 M393A4K40DB3-CWE - Samsung/Dell A4 HMA84GR7DJR4N-XN - SK hynix A5 HMA84GR7DJR4N-XN - SK hynix A6 HMA84GR7DJR4N-XN - SK hynix A7 HMA84GR7DJR4N-XN - SK hynix A8 HMA84GR7DJR4N-XN - SK hynix A9 HMA84GR7DJR4N-XN - SK hynix A10 HMA84GR7DJR4N-XN - SK hynix A11 M393A4K40DB3-CWE - Samsung/Dell A12 HMA84GR7DJR4N-XN - SK hynix  B1 HMA84GR7DJR4N-XN - SK hynix B2 M393A4K40DB3-CWE - Samsung/Dell B3 M393A4K40DB3-CWE - Samsung/Dell B4 M393A4K40DB3-CWE - Samsung/Dell B5 HMA84GR7DJR4N-XN - SK hynix B6 HMA84GR7DJR4N-XN - SK hynix B7 HMA84GR7DJR4N-XN - SK hynix B8 HMA84GR7DJR4N-XN - SK hynix B9 M393A4K40DB3-CWE - Samsung/Dell B10 HMA84GR7DJR4N-XN - SK hynix B11 M393A4K40DB3-CWE - Samsung/Dell B12 HMA84GR7DJR4N-XN - SK hynix
Memory upgrade leads to "Configuring memory" R640