RE: SharePoint Online Content in Dynamics 365 CRM form using IFrame
SharePoint Online or SharePoint Server 2013, when the page is called from the frame, if the URL of the parent window (caller) page is not in the same domain, it will block displaying the page in the frame.
Please use the AllowFraming control when you create a page that you want to allow access from the frame of another domain page.
Please use the AllowFraming control only on frames-only pages. Never use it on a master page.
Title: AllowFraming class
Address: msdn.microsoft.com/.../microsoft.sharepoint.webpartpages.allowframing(v=office.15).aspx
Procedure
1. Open the page in SharePoint Designer.
2. Add the following if there is no reference to the Microsoft.SharePoint.WebPartPages namespace in the page header.
 <%@ Register tagprefix="WebPartPages" namespace="Microsoft.SharePoint.WebPartPages" assembly="Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
3. Add the AllowFraming control to your page.
<WebPartPages:AllowFraming runat="server"/>
4. Save it by overwritting.
I recommend you to create a support ticket for further investigation if the issue is not resolved.
reference
URL:docs.microsoft.com/.../sharepoint-frameiframe