In case anyone sees "Client not found" error from Lumin app when logging into Tidal, please:

1. Upgrade Lumin firmware to the latest (if firmware upgrade is not offered, please power cycle the Lumin unit, kill the Lumin app, relaunch it, then toggle Firmware Upgrade setting to Off then On).

2. Upgrade Lumin app to the latest from Apple app store, Google Play Store, or Lumin web site for Android apk download

The cause is that Tidal service has deprecated the old username password login method, and enforced the use of the new OAuth login model.