Domain not available

Domain not available

Hello zoho team,

I've read recently in the forum that you can point a cname sub domain to business.zoho.com, so
I did it with office.<our domain>.
Yesterday every thing is fine today I get this error:

Domain not available
You do not belong to this domain.Please check your portal URL again.

Note:
  1. Our domain is already added & verified.
  2. I've tried to change the portal url to office.<our domain> from the dashboard, It says url is already taken.
Please Help!