Failed to generate a Let’s Encrypt certificate on Asustor NAS with error Ref. 5402

While trying to generate a Let’s Encrypt certificate for my Asustor NAS through Settings -> Certificate Manager (following the instructions), the process repeatedly failed with error Ref. 5402. Uninstalling/Reinstalling Let's Encrypt ACME Client from the Asustor App Central didn’t help. What else can I try