I found the problem.
I am using OPNsense and forgot to disable TX checksum offloading. Very interesting that this checksum offloading caused catastrophic network disruptions on a Realtek nic, but no noticeable performance hit on Intel nics. This was an old host that featured a Realtek card. All my recent hosts that I use have only Intel nics. That is why I forgot about the whole offloading thing.

Thanks for the tips.

Best wishes to the whole community!

@olivierlambert said in Very scary host reboot issue:

I am very very busy so I don't have time to make a search by myself but maybe someone else around with few minutes could point you to the blog post talking about this

edit: found it in few sec luckily: https://xcp-ng.org/blog/2024/01/26/january-2024-security-update/

Thanks. I'll check this out.