Dear all,
I just realized that my paste was cut off, so I provide another one
which is complete:
  Oct 28 19:49:41 v220191283267104968 systemd[1]:
Starting Knot Resolver daemon...
 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)
 Oct 28 19:49:41 v220191283267104968 kresd[22481]: [system] error while loading config:
error occurred here (config filename:lineno is at the bottom, if config is involved):
 Oct 28 19:49:41 v220191283267104968 kresd[22481]: stack traceback:
 Oct 28 19:49:41 v220191283267104968 kresd[22481]:         [C]: in function 'tls'
 Oct 28 19:49:41 v220191283267104968 kresd[22481]:
/etc/knot-resolver/kresd.conf:3: in main chunk
 Oct 28 19:49:41 v220191283267104968 kresd[22481]: ERROR: Invalid argument (workdir
'/var/lib/knot-resolver')
 Oct 28 19:49:41 v220191283267104968 systemd[1]: kresd(a)2.service: Main process exited,
code=exited, status=1/FAILURE
 Oct 28 19:49:41 v220191283267104968 systemd[1]: kresd(a)2.service: Failed with result
'exit-code'.
 Oct 28 19:49:41 v220191283267104968 systemd[1]: Failed to start Knot
 Resolver daemon. 
Best regards,
Martin