Hi sfisher_x
Not sure if you've made any progress on this but I just got this error for the first time after changing my memory to use DOCP(ASUS implementation of XMP) about a week ago.
I have a very similar set up:
CPU: Ryzen 3700x
RAM: 4 x 16GB GSkill 3200mhz
Motherboard: Asus TUF x570 plus (using BIOS 2607 14 Aug 2020)
Oct 10 02:27:43 Tower kernel: mce: [Hardware Error]: Machine check events logged
Oct 10 02:27:43 Tower kernel: [Hardware Error]: Corrected error, no action required.
Oct 10 02:27:43 Tower kernel: [Hardware Error]: CPU:0 (17:71:0) MC9_STATUS[-|CE|MiscV|AddrV|-|-|SyndV|CECC|-|-|-]: 0x9c2040000004010b
Oct 10 02:27:43 Tower kernel: [Hardware Error]: Error Addr: 0x0000000be8042290
Oct 10 02:27:43 Tower kernel: [Hardware Error]: IPID: 0x000700b020350200, Syndrome: 0x0000af411a470202
Oct 10 02:27:43 Tower kernel: [Hardware Error]: L3 Cache Ext. Error Code: 4, L3M Data ECC Error.
Oct 10 02:27:43 Tower kernel: [Hardware Error]: cache level: L3/GEN, tx: GEN, mem-tx: GEN
I too had 3 passes of memtest without errors when I tested but now a week later I've seen this error pop up.
This post was pretty informative:
So it seems the highest officially supported transfer rate for my setup is 2933Mhz (possibly the same for yours, though you haven't indicated you memory configuration).
I'm not sure how concerned we should be about these errors but I will monitor and if it recurs with any regularity I will lower my rates back in spec.