That’s a pretty odd error, I had to look it up just to see what it is. 429 is too many requests, which doesn’t sound like it should be happening on the first attempt to login.
The 429 status code indicates that the user has sent too many
requests in a given amount of time ("rate limiting").
The response representations SHOULD include details explaining the
condition, and MAY include a Retry-After header indicating how long
to wait before making a new request.
For example:
HTTP/1.1 429 Too Many Requests
Content-Type: text/html
Retry-After: 3600
<html>
<head>
<title>Too Many Requests</title>
</head>
<body>
<h1>Too Many Requests</h1>
<p>I only allow 50 requests per hour to this Web site per
logged in user. Try again soon.</p>
</body>
</html>
Note that this specification does not define how the origin server
identifies the user, nor how it counts requests. For example, an
origin server that is limiting request rates can do so based upon
counts of requests on a per-resource basis, across the entire server,
or even among a set of servers. Likewise, it might identify the user
by its authentication credentials, or a stateful cookie.
Responses with the 429 status code MUST NOT be stored by a cache.
Odd too that the second login works.
It definitely seems like a hosting issue, and you should ask Mediatemple how their server defines the user and counts requests.
You might also check your log files just to make sure hackers aren’t trying to access the site in a brute force attack that is triggering it. Still I’m not sure why the second login works.