My Website > Site URL was “www.mysitedomain.com” and my Basic Info > App domain was “mysitedomain.com” and everything was working as expected.
Just recently Facebook rolled out a new version of the app settings page.
After the new version, I modified my Basic Info > App domain for testing purposes. When I tried to revert it back to “mysitedomain.com” it complained saying “mysitedomain.com must be derived from your Site URL” (see picture).

Does anyone know if this is indeed a bug? If not, did the meaning of this two settings changed and I am missing something?
EDIT: I need my App Domain to be mysitedomain.com. So the question would be what do I need to put on Site URL.
Eventually it fixed by itself, seems there was an issue with FB. I already voted to close the question as too localized.