There is an ongoing issue with some custom domains not performing correctly. The issue does not affect all accounts. We are actively investigating to determine and fix the cause. If you need immediate access to your Mautic instance, use https://YOUR-SUBDOMAIN.mautic.net.
UPDATE 30/09/2016 15:15
This issue has been located and resolved.
As a reminder:
Today we shutdown our .com IP address (184.108.40.206).
So what does this mean? This means that wherever you reference ‘mautic.com’, you will need to change to reference ‘mautic.net’. That includes changes to the following:
In addition to the locations above, all references related to subdomains will need to change as well; from [subdomain].mautic.com to [subdomain].mautic.net.
Also, if you're using a custom domain, please update either the CNAME record to mautic.net, or A record to 220.127.116.11 to prevent a disruption of service.