As part of migrating the old jessie instance (T236553), I've created a new instance, configured it etc. The last step should have been removing the web proxy matching "phragile.wmflabs.org" wth the IP of the old instance (this part I've successfully performed) and adding the proxy to point "phragile.wmflabs.org" the the new instance.
When clicking "Create Proxy" in Horizon the error message saying "Danger: An error occurred. Please try again later." appears. Truing again later (over several hours) didn't work.
It looks the Horizon UI has problems as requesting https://horizon.wikimedia.org/project/proxy/create/results with HTTP 500, and the following response
<!DOCTYPE html>
<html>
<head>
<meta content='text/html; charset=utf-8' http-equiv='Content-Type' />
<link rel="shortcut icon" href="/static/dashboard/img/favicon.ico"/>
<link rel="apple-touch-icon" sizes="180x180" href="/static/dashboard/img/apple-touch-icon.png" />
<link rel="mask-icon" href="/static/dashboard/img/safari-pinned-tab.svg" color="#5bbad5" />
<title>Server error - Manage Wikimedia Cloud</title>
<style>
a {
color: #43a1d6;
text-decoration: none;
}
body {
color: rgb(106, 106, 106);
text-align: center;
font-weight: normal;
background: none repeat scroll 0% 0% rgb(250, 250, 250);
}
div#container {
padding-top: 170px;
margin: 72px auto 0 auto;
width: 390px;
background: url("/static/dashboard/img/logo.svg") no-repeat scroll center 35px padding-box rgb(255, 255, 255);
background-size: 320px;
border: 1px solid rgb(225, 225, 225);
-webkit-border-radius: 6px;
-moz-border-radius: 6px;
border-radius: 6px;
box-shadow: 0px 3px 7px rgba(0, 0, 0, 0.3);
-webkit-box-shadow: 0px 3px 7px rgba(0, 0, 0, 0.3);
-moz-box-shadow: 0px 3px 7px rgba(0, 0, 0, 0.3);
}
h2 {
font-weight: normal;
}
div#container > div {
padding: 25px;
}
</style>
</head>
<body id="">
<div id="container">
<div id="text">
<h2>Something went wrong!</h2>
<p>An unexpected error has occurred. Try refreshing the page. If that doesn't help, contact your local administrator.</p>
</div>
<div id="links">
<p><a href="/">Home</a></p>
<p><a href="https://wikitech.wikimedia.org/wiki/Horizon">Help</a></p>
</div>
</div>
</body>
</html>
<!-- KQOXZ85JHrRmdJZ84 -->
I am not qualified to say whether this is project-specific issue, or whether something Horizon-wide.