There is a strange thing happening I think since last updates (using now 184.108.40.206 version).
When you go to the domain mapping tool page from one of the subsites dashboard:
- if you don't have defined any mapped domain yet: the new domain form is well displayed.
- but if you have one or more mapped domain already defined, the new domain form is not displayed (so you cannot add a new mapped domain)
I don't think this is a normal behaviour, is it ?