Nextcloud Talk funktioniert nicht

Use Nextcloud Talk to have one-on-one or group audio or video calls, create or join web conferences and send chat messages.  Nextcloud is a private, self-hosted file sync and communication platform, designed to put you back in control over your data.  The Nextcloud Talk app requires a Nextcloud Talk server to function.  It runs on a server of your choice, be it at home, at a service provider or in your enterprise, and gives you access to your documents, calendars, contacts, email and other data.  Nextcloud is fully open source, giving you the option to extend them for your own use, participate in their development or simply verify they do what we promise.  You can share with others even across different Nextcloud servers and work together on documents.  Nextcloud Talk is easy to use and will always be completely free!  Millions of users use Nextcloud daily at businesses and homes around the world.  All communication is fully encrypted and mediated by your own server, providing the highest degree of privacy possible.  Business users rely on the professional support of Nextcloud GmbH, making sure they have a fully supported, enterprise-ready platform for productivity and collaboration, fully under control of their IT department.