The work of the YouTube downloading apps depends a lot on the 'rules' implemented by YouTube and Browsers. Changes can cause issues in our apps. We keep the finger on the pulse of the latest changes and prepare the working version of our downloader the sooner the better. That is why it is very important to check if your program is up to date. Very often it happens that the issue has already been resolved and the user just needs to update his app.
How to check if your version is up to date?
You can also check 'Help - Check for updates' in the program itself. BUT: we don't update this info each time we upload the new version to our website, only in very important cases. That is why it is recommended to use the website.
Download, install, and run the new version. Do you get the same error message as before? Send your email to support, mention your program version and the error you're getting.