Website authentication error during DAP downloads

A few days ago, I was trying to download videos from Youtube. I tried using Keepvid and Download Accelerator Plus which accelerates the download speed. At first, it was downloading smoothly. But after I put more and more links to download at DAP, the website authentication error popped up, which it asked me to enter the user name and password.

I began to wonder whether Youtube user can lock certain videos or set certain videos to download from his/her account or channel. I tried again by clicking Resume button on Download Accelerator Plus, but the error dialog box still popped up. Next day, it is still the same.

I tried again using ordinary download from my browser and it worked. Then, I tried downloading the same file again by using DAP but starting download from zero, not resuming the previous download. This time, it worked perfectly. Now, I know that putting lots of links on DAP probably will burden the downloader and maybe it will make Youtube to stop you from downloading like that. Now, I know that if error of authentication pops up, we should stop downloading and try downloading again at a later time by using another new link.

NB. You can also use Youtube Downloader to download your videos from Youtube.