UPDATE: The problem has been resolved. You can read more here.
A friend contacted me yesterday complaining to me that the custom domain he wanted to map to his blogspot domain keeps displaying "404 error" page even after adding the custom domain to his blog via Blogger settings.
He forwarded his Blogspot login details to me so I can help him look into the problem and when I logged in, I was surprised to see "Maintenance in progress - Domain switching disabled" under the "publishing" tab of the Blogger Basic Settings page.
I first thought he is the only one been affected but I was stunned when I checked my blog's settings and saw the same thing right there as seen in the picture below:
Without any delay, I decided to Google the error and it became obvious to me that it is a general issue with Blogger/Blogspot blogs. I felt relieved when I stumbled on a post on the Known Issues for Blogger blog where Blogger announced that they are aware of the issue and working on fixing it as soon as possible.
So, What is the Solution to the "Maintenance in progress - Domain switching disabled" error?
==> Just wait for Blogger to fix it. Blogger will surely update us once it has been fixed. You can also follow up with updates via this thread at Google Groups.
Have you noticed the same thing via your Blogger settings? Can you access your blog via your custom domain?