We recently encountered an issue after the latest SharePoint Online service update where our SharePoint Apps that make Cross-Domain calls were returning errors that they were unable to read the list data in the HostWeb.
These same Apps would run fine in other SharePoint Online Tenants but not in our Tenant. After several weeks of interaction with Microsoft the issue was identified as being a problem that was introduced by the latest Service Update and the case of our AppWebURL. Our AppWebURL was being passed as CompanyName.sharepoint.com instead of companyname.sharepoint.com. Once we modified our code to make the AppWebURL lowercase the issue was resolved. Now our code to retrieve the AppWebURL looks like this: