we see the same behaviour with the guest accounts we have created. I don't think it has something to do with the "supported account types" setting of the application itself. The problem is that https://myapplications.microsoft.com won't allow private Microsoft accounts to login. Using the link that ist mentioned in the invitation mail - https://account.activedirectory.windowsazure.com - works for us.
From there, the guest user can login and is redirected to https://myapplications.microsoft.com. If this is expected behaviour, this is very irritating and badly communicated by Microsoft (i haven't found any official document on this behavior yet).