Proxmox 401 'authentication failure': Fix Login and API Errors [2026]

You open the Proxmox web interface, enter your credentials, and nothing happens. The browser's dev tools show a red 401 in the network tab. Or worse: your automation script that worked yesterday now fails with the same code. "authentication failure" is Proxmox's most common login error — and in most cases it's not a server problem, it's a small configuration detail. Here's how to diagnose and fix…