Oh no. That's was unexpected. Can you please retry it as this could have simply been an intermittent issue. Thanks for report though.

We have opened an issue on github to investigate this further. If you can provide us more details or just want to join the progress, you can visit the issue at https://github.com/gurumukhi/youtube-screenshot/issues/81. Thanks for using the addon, I hope this issue has been resolved or will be resolved very soon and you update to a better rating. Thanks again :)