I installed from the git head.

For the callback status, it's:
(:error (error http 405) :redirect https://bpaste.net/show/9f7b83e91ebc)

For the callback buffer, it's:
HTTP/1.1 405 METHOD NOT ALLOWED
Server: nginx
Date: Sun, 21 Feb 2016 10:45:21 GMT
Content-Type: text/html
Content-Length: 178
Connection: keep-alive
Allow: HEAD, OPTIONS, GET
X-Cache-Hits: 0
X-Cache-Age: 0
X-Cache-Status: MISS

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<title>405 Method Not Allowed</title>
<h1>Method Not Allowed</h1>
<p>The method is not allowed for the requested URL.</p>

For the url-debug output, it's:
https://bpaste.net/show/f5b3e99df8fd

*****

So from the url-debug output, the logic is doing wrong. On Line#30, it's doing a *redirect* with post, resulting an HTTP 405.

BTW, possible to do nothing other than firing up the callback when receiving the HTTP 302 on Line#32 ?

Regards.