Impossible to transfer domain!? Advice?

I recently purchased a domain (.io) on Sedo. The seller has provided the auth code, so all good to go.

Initially attempted to transfer into GoDaddy, as I have done a million times before. However, GD's transfer tool simply wouldn't recognise the domain as being eligible for transfer. Nothing about it being locked or anything – just a simple message saying not eligible. Looked at whois records, and nothing in there would suggest any sort of problem (not changed registrars for years, status…

Impossible to transfer domain!? Advice?