Failed to load TLS config: No private keys found in file ...

[mildred]

Trying to use stalwart-imap, I fail to configure it regarding TLS settings.

I generated a self signed certificate and a secret key, both in PEM format, and I get from stalwart-imap the following message:

stalwart-imap[2446261]: Failed to load TLS config: No private keys found in file /etc/ssl/private/stalwart-imap.pem

I tried to point to the secret key and certificate (after granting access to the directory) to let's encrypt generated certificates (requested using Caddy) and I have the same error.

Perhaps the secret key does not have the correct format, but PEM is pretty standard and I don't see what's needed.