[object HTMLScriptElement] editing a template

I was trying to edit the default invoice template adding some static text but when I save I got the following error in a pop-up box "[object HTMLScriptElement]". After clicking OK (the only option) the save cleared the template.

I am now left without a template for invoices and left pretty unhappy.

Ay suggestions on how I can get the template back and how I can get this to work.