I’m having the same error 
ionic cordova resources android --force
√ Collecting resource configuration and source images - done!
√ Filtering out image resources that do not need regeneration - done!
- Uploading source images to prepare for transformations
HTTP Error 503: POST https://res.ionic.io/api/v1/upload
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta charset="utf-8">
<title>Application Error</title>
<style media="screen">
html,body,iframe {
margin: 0;
padding: 0;
}
html,body {
height: 100%;
overflow: hidden;
}
iframe {
width: 100%;
height: 100%;
border: 0;
}
</style>
</head>
<body>
<iframe src="//www.herokucdn.com/error-pages/application-error.html"></iframe>
</body>
/ Uploading source images to prepare for transformations