I recommend upgrading to PRO.
Quite useful at low cost.
Please note that the specifications for PRO activation have changed recently.
If you can't solve the problem, ask support once and he may help you.
Developer was a good man of feeling. I got a reply

I use this add-on everyday to get YouTube videos for offline use, I use it to populate my my personal media server [Plex] & to help others with tutorial videos at work. I have the paid version since 2018 it's worth the money for quality 1080p videos.

One of the great gifts to the internet. Thank you! Appreciate your communication about the current status each update, and the behind the scenes challenges. Helps us appreciate your work all the more!

I've been using this for years as a pro user! It is awesome. Come on guys it's only 30 once! There's no annual fee like so many programs nowadays!

Normal youtube download websites recently started only offering 360p versions of their videos with sound, or 1080p versions with no sound, so I looked up a better way to get full video + sound... and this is it! I tentatively upgraded to pro and it has worked perfectly. Thanks for dealing with Youtube's nonsense for us!
Update from a year after I started using it: still going strong! This extension is awesome!

Excellent performance. Doing great job.

the greatest downloader. nothing else is remotely close.

spend a few bucks years ago and best yt downloader in the net.