Thank you both for your responses. Good to hear the bug is already registered and someone is working on a bugfix.
I'll leave this thread open until the bugfix is released.
Since updating Proxmox Backup Server to proxmox-backup-server 4.1.2-1, the scheduled Renew Certificate task (acme-renew-cert) runs indefinitely and does not renew the certificate.
Manual renewal finishes quickly (proxmox-backup-manager acme cert...