Ok, so I removed my Spotify credentials, unistalled and reinstalled Librespot, then I've sent journalctl -u service.librespot via PuTTy and this is what I have:
Nov 07 08:44:51 CoreELEC systemd[1]: Started librespot.
Nov 07 08:44:51 CoreELEC sh[18061]: INFO:librespot: librespot 582a7b5 (2018-10-04). Built on 2018-10-04. Build ID: 4eE7k6KS
Nov 07 08:44:51 CoreELEC sh[18061]: *** WARNING *** The program 'librespot' uses the Apple Bonjour compatibility layer of Avahi.
Nov 07 08:44:51 CoreELEC librespot[18079]: *** WARNING *** The program 'librespot' uses the Apple Bonjour compatibility layer of Avahi.
Nov 07 08:44:51 CoreELEC librespot[18079]: *** WARNING *** Please fix your application to use the native API of Avahi!
Nov 07 08:44:51 CoreELEC sh[18061]: *** WARNING *** Please fix your application to use the native API of Avahi!
Nov 07 08:44:51 CoreELEC sh[18061]: *** WARNING *** For more information see <Avahi Gains Compatibility Layers for Apple Bonjour and HOWL>
Nov 07 08:44:51 CoreELEC librespot[18079]: *** WARNING *** For more information see <Avahi Gains Compatibility Layers for Apple Bonjour and HOWL>
Nov 07 08:44:55 CoreELEC systemd[1]: Stopping librespot...
Nov 07 08:44:55 CoreELEC systemd[1]: Stopped librespot.
Nov 07 08:45:00 CoreELEC systemd[1]: Started librespot.
Nov 07 08:45:00 CoreELEC sh[18102]: INFO:librespot: librespot 582a7b5 (2018-10-04). Built on 2018-10-04. Build ID: 4eE7k6KS
Nov 07 08:45:00 CoreELEC sh[18102]: *** WARNING *** The program 'librespot' uses the Apple Bonjour compatibility layer of Avahi.
Nov 07 08:45:00 CoreELEC librespot[18123]: *** WARNING *** The program 'librespot' uses the Apple Bonjour compatibility layer of Avahi.
Nov 07 08:45:00 CoreELEC sh[18102]: *** WARNING *** Please fix your application to use the native API of Avahi!
Nov 07 08:45:00 CoreELEC librespot[18123]: *** WARNING *** Please fix your application to use the native API of Avahi!
Nov 07 08:45:00 CoreELEC sh[18102]: *** WARNING *** For more information see <Avahi Gains Compatibility Layers for Apple Bonjour and HOWL>
Nov 07 08:45:00 CoreELEC librespot[18123]: *** WARNING *** For more information see <Avahi Gains Compatibility Layers for Apple Bonjour and HOWL>
Nov 07 08:45:10 CoreELEC systemd[1]: Stopping librespot...
Nov 07 08:45:10 CoreELEC systemd[1]: Stopped librespot.
Nov 07 08:45:11 CoreELEC systemd[1]: Started librespot.
Nov 07 08:45:11 CoreELEC sh[18165]: INFO:librespot: librespot eb5f246 (2018-10-24). Built on 2018-10-25. Build ID: AoGSVp99
Nov 07 08:45:11 CoreELEC sh[18165]: *** WARNING *** The program 'librespot' uses the Apple Bonjour compatibility layer of Avahi.
Nov 07 08:45:11 CoreELEC librespot[18183]: *** WARNING *** The program 'librespot' uses the Apple Bonjour compatibility layer of Avahi.
Nov 07 08:45:11 CoreELEC librespot[18183]: *** WARNING *** Please fix your application to use the native API of Avahi!
Nov 07 08:45:11 CoreELEC sh[18165]: *** WARNING *** Please fix your application to use the native API of Avahi!
Nov 07 08:45:11 CoreELEC sh[18165]: *** WARNING *** For more information see <Avahi Gains Compatibility Layers for Apple Bonjour and HOWL>
Nov 07 08:45:11 CoreELEC librespot[18183]: *** WARNING *** For more information see <Avahi Gains Compatibility Layers for Apple Bonjour and HOWL>
PS: But now, after removing my Spotify credentials, it's working!