Overview
You are experiencing a recurrent connectivity issue in which the primary link keeps dropping, even when there is not much traffic. The internet service may still be working without service interruptions. Restarting Kerio works for some time but then you face the same issue again.
You are using a virtual machine inside a Proxmox virtual environment, and the driver used for networking is VirtIO
Solution
Proxmox is not in the list of hypervisors supported by Kerio Control. However, this issue can be resolved by modifying the drivers used for the networking from VirtIO to vmxnet3.
