The reason was backup of this specific site collection did not complete successfully. When you backup a Site, SharePoint places the site in read-only mode. But since my backup hanged the site never got out the read-only mode.
- Go to Central Administration.
- Under Application Management go to "Configure quotas and locks".
- Set Site lock information to "Not locked".
Luckily since the April 2013 CU update for SharePoint 2013 we now have the SpsiteAdministration.ClearMaintenanceMode method. So we can clear the flag with the flowing two lines:
$site = new-object Microsoft.SharePoint.Administration.SPSiteAdministration(‘http://intranet.demo.local’)
and your site is out of maintenance mode again!