Microsoft Office 365 redirects clicked links in emails to a "safe links" service.  The "safe link" service will probe the link before redirecting the user, which means that if the registration is completed by the GET-request, the process is already done by the time the user loads the registration page.

The user will then be greeted with an error message that declares that the registration process failed, although the account have actually been created.

In general we should not make database modifications based on GET-requests.  I propose that the verification link should land on a page with a button which needs to be clicked to send a POST-request to complete the registration.