Thank you very much for using the addon and reporting the issue. We are looking into the matter, you can check the progress at https://github.com/gurumukhi/youtube-screenshot/issues/43.

EDIT: The issue has been resolved and new version with the fix is now available. This issue happened because of changes in YouTube's side. Thanks a lot for reporting the issue, this is the only way we can know about such issues. Kindly update the addon and enjoy the experience. Hope you give us a five star review :), feel free to let us know if you notice anything else.