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.