SSH log in to your PVE as root:

cd /usr/share/javascript/proxmox-widget-toolkit;
cp proxmoxlib.js proxmoxlib.js.bak;
nano proxmoxlib.js

Search (in Nano: Control + W) for the exact expression:

Ext.Msg.show({
title: gettext('No valid subscription'),
icon: Ext.Msg.WARNING

and change it to:

void({ //Ext.Msg.show({
title: gettext('No valid subscription'),
icon: Ext.Msg.WARNING

Logout, force „empty cache“ and reload browser and log back in.
Warning should now be gone.

Create your logo with a graphic editor of your choice.
Size of logo should be 172x30px.
Copy the logo via SCP to the home directory of your root user and SSH as root to your PVE.
Backup the original logo:

cp /usr/share/pve-manager/images/proxmox_logo.png /usr/share/pve-manager/images/proxmox_logo.png.bak

Copy and replace your logo to:

cp <your-logo-file>.png /usr/share/pve-manager/images/proxmox_logo.png

OR:
If you are very confident and you have already backuped the original logo, then just SCP the new logo directly.

scp <your-logo-file>.png root@<your.pve.IPorNAME.here>:/usr/share/pve-manager/images/proxmox_logo.png

Empty cache and force a full reload of your browser.

Enjoy!