GetSiteAndWeb access denied after Office365 service update - why?

Typically this call GetSiteAndWeb, using Office365 authentication, does not require extra authentication to succeed. This call starts failing recently at subsite level (but still ok on site collection level). Although uncertain, timing points to an Office365 service update. And I have no visibility to whether there is additional admin change.

• What are the possible reason(s) for access denied for this call to start failing?


• Any suggested action to troubleshoot this?

Request:

POST http://ift.tt/1NFN5Mz HTTP/1.1
User-Agent: myApp/7.1
Content-Type: text/xml; charset=utf-8
SOAPAction: "http://ift.tt/1FH1lyV"
Host: mysite.sharepoint.com
Content-Length: 439
Expect: 100-continue
Connection: Keep-Alive

<?xml version="1.0" encoding="utf-8"?><soap:Envelope xmlns:soap="http://ift.tt/sVJIaE" xmlns:xsi="http://ift.tt/ra1lAU" xmlns:xsd="http://ift.tt/tphNwY"><soap:Body><GetSiteAndWeb xmlns="http://ift.tt/rrg8m0"><strUrl>http://ift.tt/1FH1lyX;

Response:

HTTP/1.1 403 Forbidden
Cache-Control: private
Content-Type: text/plain; charset=utf-8
Server: Microsoft-IIS/7.5
X-SharePointHealthScore: 0
X-Forms_Based_Auth_Required: http://ift.tt/1NFN4Iv
X-Forms_Based_Auth_Return_Url: http://ift.tt/1FH1oe1
X-MSDAVEXT_Error: 917656; Access+denied.+Before+opening+files+in+this+location%2c+you+must+first+browse+to+the+web+site+and+select+the+option+to+login+automatically.
X-AspNet-Version: 4.0.30319
SPRequestGuid: ec72f19c-20dd-1000-b497-23bac5a0fc1a
request-id: ec72f19c-20dd-1000-b497-23bac5a0fc1a
X-FRAME-OPTIONS: SAMEORIGIN
SPRequestDuration: 35
SPIisLatency: 1
X-IDCRL_AUTH_PARAMS_V1: IDCRL Type="BPOSIDCRL", EndPoint="/sites/projects/_vti_bin/idcrl.svc/", RootDomain="sharepoint.com", Policy="MBI"
X-Powered-By: ASP.NET
MicrosoftSharePointTeamServices: 16.0.0.3722
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: Tue, 10 Mar 2015 18:45:56 GMT
Content-Length: 13

403 FORBIDDEN