Symptoms
On attempt to purchase an Office365 subscription passing a non-onmicrosoft.com subdomain to the "Use an existing Microsoft account" section, screen hangs and the following error appears in the browser console as a response to POST .../getDomainOwnershipDetails request:
500 (Internal Server Error)
Cause
Office365 Provider's Guide explicitly states:
"As a user of your Microsoft account, you have a user ID similar to johnsmith@example.onmicrosoft.com."
It is not possible to use a non-onmicrosoft.com subdomain currently.
Resolution
Feature Request #PFR-555: "Unable to place order for existing Microsoft customer with a different email address other onmicrosoft.com" has been addressed to Odin Development team in order to implement this functionality.
Software-related issue #APSA-18841 has been submitted in order to implement error reporting of subdomain form and return some clear warning on attempt to use a non-onmicrosoft.com subdomain.