Have a strange thing having trouble tracking down.
Installed multi-domain with Buddypress. Added an extra domain for blog choice, aside from the main domain.
If I go to the front-end (as an admin or user) and choose "Create a site!" I see all the correct options. I can add to domain1.com or domain2.net.
If I add to domain1.com everything works fine.
If I add to domain2.net, it processes, then delivers the message your blog at xxx.domain1.com is successful!
If I go to xxx.domain1.com, sure enough, it's there and works. It will not create the domain at xxx.domain2.net, it will switch it to be domain1.com.
However, if I go to the dashboard and switch to network admin, and try the exact same process it works perfectly. It will correctly add xxx.domain2.net. You can go to the domain and it works great.
So, obviously the plugin is working. It's functional, just not for front-end users.
Any idea what this could be?