Nextcloud Talk: cannot find user

Hi, I had a problem when starting a new conversation:
Somehow the list of available users in our NC instance (NCv24.0.7) is limited

  • either I see users from A-L or
  • from L-Z
    This effect shows up with Android (latest version) but also with IOS.
    Anybody has an idea what goes wrong?
    Best regards - Pingus.