A family of Microsoft word processing software products for creating web, email, and print documents.
Usually with Word problems safe mode is suggested but that does not address your problem. You could try renaming your normal.dotm file to see if that is the source of the problem.
Does your Add-In modify the Ribbon using XML in the template or does it do it via vba?
I am willing to work on this with you but would need your .dotm. I am not able to do much more today because I am due in court.
If you want, save a sample document with the problem on OneDrive or Dropbox and paste a link here. Remove any confidential or sensitive information first. I or someone else can take a look at it.