Sophos Home Firewall is rebooting multiple times a day.

Proxmox VE is a very stable platform, so I would guess this has other reasons. Can you exclude external factors form your power source or LAN?

It is small PC with 6x 2.5GBe etnernets.

THis model: https://pl.aliexpress.com/item/1005004336924039.html?spm=a2g0o.order_list.0.0.21ef1c24rPWVq3&gatewayAdapt=glo2pol

It was working with OPNSense for over 3 months without any issues. I decided to move to Sophos and issues begin.

It is small PC with 6x 2.5GBe etnernets.

THis model: https://pl.aliexpress.com/item/1005004336924039.html?spm=a2g0o.order_list.0.0.21ef1c24rPWVq3&gatewayAdapt=glo2pol

It was working with OPNSense for over 3 months without any issues. I decided to move to Sophos and issues begin.

So are you running virtualized or non-virtualized at the moment?

If situation does not change running either way, I almost certainly say it does not derive from the sophos OS.

I was using OPNSense (Bare Metal), then switch to Proxmox with OPNSens (Virtual).

Next I decided to move to Sophos (KVM Virtual Appliance) and have issue with it.

I tested on newest version 19.0.1 MR1 and 18.5.4 MR-4.

I added Zabbix monitoring to the system and I don't see any CPU/memory peaks.

Can you remove Proxmox and run the Sophos "Software-Installer" on bare metal?

Not possible, Sophos not supporting Intel i225 card.

Problem was with additional OPNSense VM. VM has 1 LAN interface only and was used as CA. After shut down this VM sophos is working stable.