One thing that can cause is due to blocking of third-party cookies and data. Here are some workaround steps to help resolve this issue:

1)  Google Chrome:
Verify that you do not have "Block third-party cookies and site data" checked in the Settings. 
  • Type chrome://settings/content
  • Under the "Cookies" header, there is a check box labeled "Block third-party cookies and site data" that should be unchecked in order for ZMC to work correctly.

2) Firefox:
Make sure that add-on or extension that is blocking third-party cookies are disabled. To view what extensions and add-ons are installed: 
  • Click the FireFox menu in the top left corner.
  • Click Add-ons.
  • Click Extensions and note any extensions that may be blocking cookies.

3) Internet Explorer
Please follow the steps and check if you have the correct settings.
  • Click Tools and then Internet Options. 
  • Go to the Privacy tab and click the Advanced button.
  • Select Accept under "Third-party Cookies". 
  • Click the OK button to apply these settings.