Exposure of Private Information in Nextcloud Server 16.0.1 causes the server to send it's domain and user IDs to the Nextcloud Lookup Server without any further data when the Lookup server is disabled.
https://nextcloud.com/security/advisory/?id=NC-SA-2019-016
https://hackerone.com/reports/508490
http://lists.opensuse.org/opensuse-security-announce/2020-02/msg00022.html
http://lists.opensuse.org/opensuse-security-announce/2020-02/msg00019.html