Nextcloud Android app is the Android client for Nextcloud, a self-hosted productivity platform. Prior to version 3.19.1, users can bypass a lock on the Nextcloud app on an Android device by repeatedly reopening the app. Version 3.19.1 contains a fix for the problem. There are currently no known workarounds.
https://hackerone.com/reports/1450368
https://github.com/nextcloud/security-advisories/security/advisories/GHSA-32j4-9xf3-h2mg