Exchange 2007 to 2013 Coexistence Issues during migration

Hi All,
I am currently in the process of migrating exchange 2007 to 2013. Everything has gone ok so far, but I am stuck on issues with coexistence.
A very brief rundown of my setup:
Single domain, single exchange 2007 server running all roles.
TMG Firewall
Single exchange 2013 server
Situation:
Exchange 2013 is mail.domain.com
Exchange 2007 is now legacy.domain.com
All relevant names exist in the UCC SSL certificate
OWA:
User with mailbox on exchange 2013 goes to mail.domain.com which is pointed to TMG. Logs in fine and opens the users mailbox on 2013.
User with mailbox on exchange 2007 goes to mail.domain.com, logs into TMG, immediately get “webpage cannot be displayed” Under more information is say )HTTP 500 internal server error)
User with mailbox on exchange 2007 goes to legacy.domain.com, logs into tmg and is correctly redirected to the exchange 2007 mailbox.

Active Sync:
User with mailbox on exchange 2013 works fine pointing to mail.domain.com
User with mailbox on exchange 2007 no longer works after changed 2007 to the legacy namespace.
In exchange 2007 I have left the external url blank for ActiveSync.

Outlook anywhere:
Users with mailbox on 2013 work fine, 2007 don’t.

Internal outlook clients:
Both with mailboxes on either server work fine.
Any help would be much appreciated.
Thanks in advance.