I ll post the answer to my problem in case any noob like me searches for it:
If you are having problems restoring a backup in domoticz addon in Libreelec:
1. disable the domoticz addon from the kodi UI
2. Rename your desired back up file to domoticz.db and copy it in the domoticz' config directory. See this post how to do this using the terminal, but it can also be done using samba/smb.
2. change permissions of the file to -rwxrw-r-- by using this command:
chmod 644 domoticz.db
3. Set the ownership to user:nobody, group: users. This is achieved by using the following commands:
chown nobody: domoticz.db
chown :users domoticz.db
4. Delete the following files in the config directory: domoticz.db-wal, domoticz.db-shm. This is essential and but it isn't mentioned anywhere as far as I know.
5. Reboot (?) and enable the addon.