I have encountered similar 942 - Memory Training Errors with various Codes (3014, 301C, 1501) several times since year 2013 in my own Z820 Workstation.
In all cases the cause was in the 8GB ECC memory sticks (32GB in all). When replaced (one by one over 3 years) with good original HP sticks, the errors ceased to appear until the next stick went bad in its turn.
In all cases the appearance of 942 - Memory Training Errors at computer startup was accompanied by a huge number of Event 47 warnings referring to Memory Machine Corrected Errors in Windows 7 System event viewer, as well as Event 20 WHEA Kernel errors. (WHEA means Windows Hardware Errors).
At some instances the faulty 8GB sticks failed to appear in System Information, leaving only 24GB active, but in most cases they remained active even though they continued to cause Events 47 and 20 during Windows operation.
In all cases the memory tests using HP Vision Memory Diagnostics Offline Edition for CD (latest edition is version 2.22.1) did not discover the faulty memory sticks.
The only remedy was to replace with good memory sticks of the same type (there are several manufacturers but they all carry the same HP part number), so HP Support should pick good memory sticks and test on-site before deciding that the errors are gone. (Assuming that your Z820 is still under warranty or a valid service agreement).
I know that there have been similar reports over the years referring to '942' errors in Z420, Z620 and Z820, and in some cases they blamed the slot/socket rather than the stick. If that is the case as you hint in your description then the motherboard may be at fault.
If there are still unused memory slots in your Z820 motherboard, you may try to move the last 8GB stick to another slot even if it is not recommended as far as symmetrical placement of memory sticks in memory channels is concerned. Again, HP Support should be consulted.