It happened many times by me in YouTube.

Imgflip is having problems with YouTube right now.

I once saw an error which said that YT was restricting video downloads, so the GIF Maker couldn't extract too many videos from there. It told me to wait some time, and then try it again, or to get the same video from a different place. It also said that I needed to buy Imgflip Pro to use it with no limits.

Right now, it's just this "Invalid URL" error though. I never got the other error again.