try to upgrade the kernel. it helped me in my case.
apt-get dist-upgrade
I think Im running the latest on the no-subscription REPO(4.15.17-1-pve). could you share your kernel version?
proxmox-ve: 5.2-2 (running kernel: 4.15.17-1-pve)
pve-manager: 5.2-1 (running version: 5.2-1/0fcd7879)
pve-kernel-4.15: 5.2-1
pve-kernel-4.15.17-1-pve: 4.15.17-9
corosync: 2.4.2-pve5
criu: 2.11.1-1~bpo90
glusterfs-client: 3.8.8-1
ksm-control-daemon: 1.2-2
libjs-extjs: 6.0.1-2
libpve-access-control: 5.0-8
libpve-apiclient-perl: 2.0-4
libpve-common-perl: 5.0-31
libpve-guest-common-perl: 2.0-16
libpve-http-server-perl: 2.0-8
libpve-storage-perl: 5.0-23
libqb0: 1.0.1-1
lvm2: 2.02.168-pve6
lxc-pve: 3.0.0-3
lxcfs: 3.0.0-1
novnc-pve: 0.6-4
proxmox-widget-toolkit: 1.0-18
pve-cluster: 5.0-27
pve-container: 2.0-23
pve-docs: 5.2-3
pve-firewall: 3.0-8
pve-firmware: 2.0-4
pve-ha-manager: 2.0-5
pve-i18n: 1.0-5
pve-libspice-server1: 0.12.8-3
pve-qemu-kvm: 2.11.1-5
pve-xtermjs: 1.0-5
qemu-server: 5.0-26
smartmontools: 6.5+svn4324-1
spiceterm: 3.0-5
vncterm: 1.5-3
zfsutils-linux: 0.7.8-pve1~bpo9