Have had three systems (two 5800x, one 3600x) that reboots/freezes due to WHEA errors. Started after about 3years problem free. One of the 5800xs so frequently it was trashed.

I wonder if it's related to the main board... Are you running X or B series chipsets? I've found that less is more when it comes to stability. Vendors always add features to justify the $200-300 price of X series.

I have always run B series because I've never needed the overclocking or additional peripherals. In my server builds I usually disable peripherals in the UEFI like Bluetooth and audio as well.