Don't use apt upgrade: https://lists.proxmox.com/pipermail/pve-devel/2025-March/068874.html
This is often caused by helper scripts. Give this a try: https://forum.proxmox.com/threads/proxmox-ve-web-gui-is-a-blank-page.172804/#post-806653
Today, we announce the availability of a new archive CDN dedicated to the long-term archival of our old and End-of-Life (EOL) releases.
Effective immediately, this archive hosts all repositories for releases based on Debian 10 (Buster) and older...