I am using DocGen version 14.4.0, Salesforce classic and have experienced the same issue in both FireFox and Chrome.
I have a complete Document Package that works perfectly. However, the requestor has asked that I go back and add some additional text instructions to the form within the package. When I edit the form and select the Add Note button the window appears for me to add my text, however, clicking OK or Cancel does nothing. I have to refresh the page to get the box to go away.
Are you able to send a screen shot of the issue by chance?
Also - Are you willing to update the app to the current AppExchange package version and see if the issue persists?