Does this occur on all browsers? If the issue happens on some browsers only, you could also try to clear the local cache of the affected web browser and see if that fixes the issue.
Do you have the Custom Chart Widgets Extension version 2.0.4 installed? If yes, could you please confirm if your Tomcat has the JAVA encoding configured to be UTF-8? That extension includes some special characters in "CombinedExtension.<timestamp>.js" that require that JAVA encoding to be enabled (additionally, it is a recommendation available in our ThingWorx Install Guide).
Please have a look at the following article (please note that even if the article is focused on a "chart doesn't exist error", I have seen this root cause affecting other widgets, like for example Google Maps... therefore it might be worthy to confirm if it is affecting also your "Note" widget):
I hope that helps!