When using an ACME certificate, it's necessary to use the domain name associated with the certificate rather than the IP address of the PBS server.
Due to Proxmox frequently querying the DNS server (where attempts to resolve my PBS server's name constituted approximately 70% of my network's DNS requests) I opted to revert to a self-signed certificate.
This was done by removing my Let's Encrypt certificate and configuring the fingerprint of the self-signed certificate in Proxmox Storage.