If you can't login on my demo (for example with Safari), that's due to your browser settings. In the Preferences, search for something like "Website tracking" and uncheck "Prevent cross-site tracking".

You can use the following accounts :

Adresse e-mail = demo@demo.com / Mot de passe = demodemo
Adresse e-mail = demo@demo.fr / Mot de passe = demodemo
Adresse e-mail = demo@demo.it / Mot de passe = demodemo
Adresse e-mail = demo@demo.es / Mot de passe = demodemo
Adresse e-mail = demo@demo.de / Mot de passe = demodemo