QNAP has been updated the new version to solve this problem, and please update the mobile app again. Sorry for the inconvenience. For more detail, please refer to the following link, https://www.qnap.com/en/how-to/faq/article/qsync-pro-for-android-cannot-connect-to-the-nas-after-updating-qsync-central-to-latest-4-4-0-6-20220301