I believe what's happening here is that we check that your Google Apps domain matches the server you're accessing Shortlinks through. So if your Google Apps Domain is '
incub.io' and your Google Apps account is ...@
incub.io then you can't access Shortlinks for
m.incub.io. This may seem too strict, but, in general, we can't assume that subdomains belong to the same organization.
The only other thing I can think of - I am not sure how your Google Apps configuration looks like: if you had a separate Google Apps domain, on
m.incub.io, with accounts like '...@
m.incub.io', then you would be able use Shortlinks on a subdomain of that. I don't think there's anything else we can do.