Monkeyslayer,
The fact that both sets of memory work on their own means the problem is not the RAM itself or probably the slots, but it does suggest the possibility that these are mixed RAM types in some detail. The 'L" means it will run at a lower power when possible and I've never known any system to use that except servers. Workstations want the RAM to run at full speed all the time. Low power and regular can't work together as the RAM can not run at two different speeds on the same system. The other possibility is that you are mixing ECC unbuffered and registered, which also will not work. The z820 will run unbuffered, but on one processor and for two CPU's it has to be registered as the one bit parity check syncs the memory for the two processors.
It's always better to use exactly the memory that arrived with the system. I have purchased used RAM, but I always buy the exact module that HP supplied and that has the identical HP part number on it. Workstations, especially dual processor ones can be quite fussy with the memory.
I don't think the BIOS version is at fault as 3.68 was the first version tthat recognized M.2 (AHCI) drives- it's reasonably modern. I had a very expensive episode with v. 3.92 on a z620 and I recommend solving absolutely everything else with your upgrade first and then approach the BIOS upgrade with the greatest care. Do not install through Windows. V. 3.92 appears to not be reversable without extreme measures and so I use 3.91 on all my zX20 systems.
Let us know what happens.
BambiBoomZ