I have finally managed to resolve the issue, which was caused by the lack of an intermediate certificate.
The ZIP file I downloaded contained the following files:
certificate.crt: This file contains the primary SSL certificate.
ca_bundle.crt: This file contains only one intermediate certificate (ZeroSSL CA). The root certificate that signs this immediate certificate is trusted by all browsers and almost all other SSL clients.
private.key: This file contains the private key, which will need to be uploaded to your server.
However, the DevilWEB 2.0 interface only supports importing a single CRT file, and cannot integrate the ca_bundle.crt file.
Therefore, I merged the two CRT files, combining the information from both into a single CRT file (I inserted the information from certificate.crt first, and then the information from ca_bundle.crt).