Right, so I’m trying to develop some Remote Event Receivers. I’ve been using our company’s tenant. Lets call it https://mytenant.sharepoint.com. That tenant was a SP2010 one, which has been upgraded.
When I try to add a new Remote Event Receiver to my project, I am prompted to log in to Office 365, presumably to https://mytenant.sharepoint.com/sites/AWBTest. I do so, and I’m shown an error page: Yup, a SP2010 error page. The path to this page is https://mytenant.sharepoint.com/_layouts/error.aspx . So it appears that visual studio doesn’t check the actual site collection that I want to use, but merely the ‘root’ site collection, and as that is in SP2010 mode, it can’t do whatever the login requires.
When I repeat this with a test SP2013-based tenant, it seems fine.