Hello.
On 29/10/2021 09.46, Martin Dosch wrote:
  Oct 28 19:49:41 v220191283267104968 kresd[22481]:
[tls]
gnutls_certificate_set_x509_key_file(/etc/letsencrypt/live/mdosch.de/fullchain.pem,/etc/letsencrypt/live/mdosch.de/privkey.pem)
 failed: -64 (GNUTLS_E_FILE_ERROR)  
I think you don't have file permissions right.  By default kresd runs
with knot-resolver:knot-resolver (user:group).
--Vladimir