Try disabling Spectre and Meltdown mitigations to see if you notice a difference, you can do that by adding mitigations=off to syslinux.cfg tne reboot, like so:
Upgrading to v6.10 and switching to ipvlan should fix it (Settings -> Docker Settings -> Docker custom network type -> ipvlan (advanced view must be enabled, top right)).
Fix the RAM problem, an uncorrectable error will halt the server.
Memtest doesn't detect ECC corrected errors, removed one or more DIMMs and see if the SEL errors stop.
Don't change the IP, see if the server gets one from the DHCP server, if it doesn't there's a problem there, or with how the serve is connecting to it.