This article details a troubleshooting process for a Proxmox cluster authentication failure that prevented users from logging into the web interface. The root cause was identified as a missing /etc/pve/access.cfg file, crucial for user authentication, likely due to corosync communication issues. The problem was resolved by restarting the pve-cluster service, forcing quorum, manually creating a minimal access.cfg file, and restarting the pveproxy service. Preventative measures include regular cluster health checks, automated configuration file backups, ensuring proper shutdown procedures, maintaining network stability, monitoring storage health, applying Proxmox updates, and careful permissions management.