We have been running Proxmox 6.0 in some servers and after a few weeks, they were having a hung issue. one-by-one all VMs would be inaccessible and the node itself will accessible in SSH only. When this article published the latest version is 6.1 but this tutorial will upgrade whatever the latest version of 6.x

How to fix it: You have to upgrade your Proxmox 6.0 into 6.1, the following are the steps:
Update a running Proxmox Virtual Environment 6.x to latest 6.x
Here is the howto for the CLI:
Check your sources.list file, should look like this:
deb http://deb.debian.org/debian buster main contrib deb http://deb.debian.org/debian buster-updates main contrib # security updates deb http://security.debian.org buster/updates main contrib
The enterprise repository:
cat /etc/apt/sources.list.d/pve-enterprise.list
deb https://enterprise.proxmox.com/debian/pve buster pve-enterprise
Update your repository and packages:
apt update
If you get any errors, your sources.list (or your network) has a problem.
Now upgrade the packages:
apt dist-upgrade
Reboot to activate the new Kernel, to check if you got all packages, run ‘pveversion -v’ and compare your output (all packages should have equal or higher version numbers):
pve-server:~# pveversion -v proxmox-ve: 6.1-2 (running kernel: 5.3.18-2-pve) pve-manager: 6.1-8 (running version: 6.1-8/806edfe1) pve-kernel-helper: 6.1-7 pve-kernel-5.3: 6.1-5 pve-kernel-5.3.18-2-pve: 5.3.18-2 ceph-fuse: 12.2.11+dfsg1-2.1+b1 corosync: 3.0.3-pve1 criu: 3.11-3 glusterfs-client: 5.5-3 ifupdown: 0.8.35+pve1 ksm-control-daemon: 1.3-1 libjs-extjs: 6.0.1-10 libknet1: 1.15-pve1 libpve-access-control: 6.0-6 libpve-apiclient-perl: 3.0-3 libpve-common-perl: 6.0-17 libpve-guest-common-perl: 3.0-5 libpve-http-server-perl: 3.0-5 libpve-storage-perl: 6.1-5 libqb0: 1.0.5-1 libspice-server1: 0.14.2-4~pve6+1 lvm2: 2.03.02-pve4 lxc-pve: 3.2.1-1 lxcfs: 3.0.3-pve60 novnc-pve: 1.1.0-1 proxmox-mini-journalreader: 1.1-1 proxmox-widget-toolkit: 2.1-3 pve-cluster: 6.1-4 pve-container: 3.0-22 pve-docs: 6.1-6 pve-edk2-firmware: 2.20200229-1 pve-firewall: 4.0-10 pve-firmware: 3.0-6 pve-ha-manager: 3.0-9 pve-i18n: 2.0-4 pve-qemu-kvm: 4.1.1-4 pve-xtermjs: 4.3.0-1 qemu-server: 6.1-7 smartmontools: 7.1-pve2 spiceterm: 3.1-1 vncterm: 1.6-1 zfsutils-linux: 0.8.3-pve1 pve-server:~#
Important Note
If you already use your Proxmox VE Subscription Key, access to the Enterprise repository works out of the box. For more details about the Proxmox VE repositories, see Package repositories