The top slot on the board (D0 if I remember correctly) was not recognized to be filled which I thought as seating problem, but persisted after resocketing both CPU and a few DIMMs and a few people suggested that cause to me. Could be faulty CPU/board, but I am trying to avoid shipping it back to China as much as I can.
I had a similar issue a few years ago when I purchased the board via retail channels. A specific DIMM slot would frequently fail and almost never come back, so I sent the board to TYAN and they did not find any problems. Unfortunately, the TOMCAT took too long to be returned to me from TYAN, so I moved on to a Supermicro H12 and haven't used the TOMCAT since 2020 yet I frequently dream of it, so I got a CPU for it today!
Back then, I tried different memory, re-seating the CPU, settings, etc... this was not an initial problem with the motherboard.
I'm not sure if the PMU Training (FFE Write Traning, DFE Read Traning, etc.) is exposed in the BIOS -- perhaps you will have some luck tweaking those settings.
I assume the CPU is the offender as the Supermicro H11 and H12 boards were more forgiving -- the CPU and memory was ALMOST always stable with the Supermicro boards -- on a rare occasion a module would dropout during a reboot, but would easily comeback up if re-rebooted.
My specific error was: "WARNING: P0_UMC2_CH_D0 has failed. It has been disabled." :
Does your board throw any errors during post?
I must point out: I did try two different processors.