Tried the steps listed on
https://developers.google.com/+/hangouts/button
Troubleshooting Hangouts
We're having trouble connecting with the plugin. Please follow our troubleshooting steps:
1. Refresh page.Done.
2. Reinstall plugin.
2.1 Uninstall plugin.
To uninstall in Linux:
Go to Applications > Debian > Applications > System > Package Management > Synaptic Package Manager.
Search for "Google Talk Plugin" in the list of packages.
Select the "Google Talk Plugin package" checkbox.
To completely uninstall (including configuration files), select "Mark for Complete Removal." To uninstall while saving your preferences, select "Mark for Removal."
Click Apply.
Select Google Talk Plugin from the "To be completely removed" menu.
Click Apply.
Refresh page. Refresh
2.2 Install plugin.Pending.