Keep Getting Failure When Using Blog Clients

I have two WordPress installations on one web hosting account; one is for the main domain, another is in an add-on domain. Using either Ecto or MarsEdit, the main account works fine, but I can do nothing with the add-on account. I updated the WordPress blog in the add-on, I tried deactivating the plug-ins, I even substituted the XMLRPC PHP code from the working account (then set it back). Nothing worked. My web host swears up and down it’s not them, not mod_security, nothing on their end as far as they can tell. But when either app tries to do anything on the blog, the response code is just a period “.”

Can anyone help, suggest a fix? Below is the log info from both clients. Thanks!

Ecto:

Request with URL:
https://[username:password@domain]/xmlrpc.php
and data:
<?xml version="1.0" encoding="UTF-8"?>
<methodCall>
<methodName>metaWeblog.getRecentPosts</methodName>
<params>
	<param>
		<value><string>1</string></value>
	</param>
	<param>
		<value><string>[username]</string></value>
	</param>
	<param>
		<value><string>[password]</string></value>
	</param>
	<param>
		<value><i4>50</i4></value>
	</param>
</params>
</methodCall>

Response:
.

MarsEdit:

Network message sent: 2014-11-21 15:03:45 +0000
URL: https://[domain]/xmlrpc.php
Method name: metaWeblog.getRecentPosts
Network reply received: 2014-11-21 15:03:46 +0000
URL: https://[domain]/xmlrpc.php
Method name: metaWeblog.getRecentPosts
Status code: 402
Succeeded: NO
--Download Error--
Request text:
<?xml version="1.0" encoding="utf-8"?>
<methodCall>
	<methodName>metaWeblog.getRecentPosts</methodName>
	<params>
		<param>
			<value><string>1</string></value>
			</param>
		<param>
			<value><string>[username]</string></value>
			</param>
		<param>
			<value><string>[password]</string></value>
			</param>
		<param>
			<value><int>30</int></value>
			</param>
		</params>
	</methodCall>

Response text:
.