My experiences with the MZ73-lm0 and MZ73-lm1 motherboards are not the best.
Error 28 with two processors occurs when two identical processors of different versions are installed.
For example, if you install two Epyc 9124 processors, one with OPN 100-000000802 and the other with OPN 100-000000916, the motherboard will freeze with error 28 (Check the BMC).
To work, two processors with the same OPN must be installed. I hope this helps.