Adding feed that requires Authentication returns NSXMLParserErrorDomain error 4.

[jimmywim]

MacOSX 10.8.2

Attempting to add an RSS feed that requires a username and password yields this error:

The feed URL you entered appears to be invalid.

Error Domain=MWFeedParser Code=3 "The operation couldn’t be completed. (NSXMLParserErrorDomain error 4.)" UserInfo=0x7ff5794fac70 {NSLocalizedDescription=The operation couldn’t be completed. (NSXMLParserErrorDomain error 4.)}

Output response from Charles when hitting Ok...

HTTP/1.1 403 Forbidden
Cache-Control: private
Content-Type: text/plain; charset=utf-8
Server: Microsoft-IIS/7.5
X-SharePointHealthScore: 6
X-Forms_Based_Auth_Required: https://XXXXXXXXXX.sharepoint.emea.microsoftonline.com/_forms/default.aspx?ReturnUrl=/_layouts/15/error.aspx&Source=%2f_layouts%2f15%2flistfeed.aspx%3fList%3d%257B845B9DC1-8C5D-429D-BE9E-2EA7A471B6EF%257D
X-Forms_Based_Auth_Return_Url: https://XXXXXXXX.sharepoint.emea.microsoftonline.com/_layouts/15/error.aspx
X-MSDAVEXT_Error: 917656; Access denied. Before opening files in this location, you must first browse to the web site and select the option to login automatically.
X-AspNet-Version: 4.0.30319
SPRequestGuid: 84f83f9c-4d2b-d08a-8788-c0344bac71e8
request-id: 84f83f9c-4d2b-d08a-8788-c0344bac71e8
X-FRAME-OPTIONS: SAMEORIGIN
SPRequestDuration: 120
SPIisLatency: 3186
X-Powered-By: ASP.NET
MicrosoftSharePointTeamServices: 16.0.0.1922
X-Content-Type-Options: nosniff
X-MS-InvokeApp: 1; RequireReadOnly
P3P: CP="ALL IND DSP COR ADM CONo CUR CUSo IVAo IVDo PSA PSD TAI TELo OUR SAMo CNT COM INT NAV ONL PHY PRE PUR UNI"
Date: Thu, 05 Sep 2013 09:02:05 GMT
Content-Length: 13

403 FORBIDDEN

Update: This only appears to happen on Microsoft Office 365-hosted SharePoint sites, it works fine on an NTLM-protected self-hosted SharePoint site (it prompts for credentials).