If you have turned on the option for the Message Centre to access files from either Dropbox or OneDrive, when you start the Message Centre you need to complete the login process to either Dropbox or OneDrive within three minutes otherwise the connection window is stopped.  

If you try to use the login to Dropbox or OneDrive after the three minute period you will receive an 'Unable to Connect' or similar notification.

To enable the connection, close the browser tab, and refresh the 'My Mail' tab in the Message Centre by clicking on it, or restart the Message Centre.  You will then be again presented with the opportunity to login to Dropbox or OneDrive again.

The function in SPM/PHM to turn on the ability to connect to Dropbox or OneDrive is available in Setup > Personnel > Operators > Message Options.