Why am I seeing net::ERR_CLEARTEXT_NOT_PERMITTED errors after upgrading to Cordova Android 8?

The default API level in the Cordova Android platform has been upgraded. On an Android 9 device, clear text communication is now disabled by default.

To allow clear text communication again, set the android:usesCleartextTraffic on your application tag to true:

<platform name="android">
  <edit-config file="app/src/main/AndroidManifest.xml" mode="merge" target="/manifest/application">
      <application android:usesCleartextTraffic="true" />
  </edit-config>
</platform>

As noted in the comments, if you have not defined the android XML namespace previously, you will receive an error: unbound prefix during build. This indicates that you need to add it to your widget tag in the same config.xml, like so:

<widget id="you-app-id" version="1.2.3"
xmlns="http://www.w3.org/ns/widgets" 
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:cdv="http://cordova.apache.org/ns/1.0">