You need to close all the sheets opened at your end and then clear your cache memory.
1. Click Wrench icon (at the top right of the browser)..>Select the option Tools..>Click ‘Clear Browsing Data’..>Mark ‘Empty the cache’ option..>Click the button ‘Clear Browsing Data’
2. The keyboard shortcut is Shift+Ctrl+Delete.
Once you have done the same, please open a single sheet and then try to send the messages.