I have just found the mistake I made in one of the last steps of the configuration.

I had written "internal_hashed" as "Internal_hashed" in the file /etc/prosody/conf.d/your_domain.cfg.lua

VirtualHost "your_domain"

??????? -- enabled = false -- Remove this line to enable this host
??????? -- authentication = "anonymous"
??????? authentication = "internal_hashed"

Now the authentification window pops up.

After the successful authentication the meeting room appears.

Kind regards,

Bernd