Issue
When using the 'Web Request' action in Nintex Workflow for Office 365, the following error may occur:
Response: 403 FORBIDDEN
["X-SharePointHealthScore=4","SPRequestGuid=e5a6fe9e-2083-1000-2cd5-f703fa2b38b0","request-id=e5a6fe9e-2083-1000-2cd5-f703fa2b38b0","MS-CV=nv6m5YMgABAs1fcD+is4sA.0","Strict-Transport-Security=max-age=31536000","SPRequestDuration=54","SPIisLatency=1","X-Forms_Based_Auth_Required=https:\/\/site url\/_forms\/default.aspx?ReturnUrl=\/_layouts\/15\/error.aspx&Source=%2fsites%2f_vti_bin%2flists.asmx","X-Forms_Based_Auth_Return_Url=https:\/\/site url/_layouts\/15\/error.aspx","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-IDCRL_AUTH_PARAMS_V1=IDCRL Type=\"BPOSIDCRL\", EndPoint=\"\/sites\\/_vti_bin\/idcrl.svc\/\", RootDomain=\"sharepoint.com/<http://sharepoint.com/>", Policy=\"MBI\"","MicrosoftSharePointTeamServices=16.0.0.19214","X-Content-Type-Options=nosniff","X-MS-InvokeApp=1; RequireReadOnly","Connection=close","Content-Length=13","Cache-Control=private, max-age=0","Content-Type=text\/plain; charset=utf-8","Date=Wed, 28 Aug 2019 12:44:27 GMT","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\"","Server=Microsoft-IIS\/10.0","X-AspNet-Version=4.0.30319","X-Powered-By=ASP.NET"]
Resolution
Use the Office 365 user account as the user credentials that have sufficient permission rather than use a Federate or Corporate account.