Yes, Youtube did change the strucutre of their website. Other Youtube download applications stopped working, too. e.g. also Canola can't download Youtube videos anymore.

In line 126 the youtube-dl-x script is trying without success to find some url matching "re.compile(r', "t": "([^"]+)"')" on the youtube page.

There is a "youtube-dl" script available at http://bitbucket.org/rg3/youtube-dl/wiki/Home which has recently been updated and is able again to download videos from Youtube.
Unfortunately this youtube-dl script and the youtube-dl-x script used by mytube seen to be quite different. I did already try to figure out how to apply the change done in youtube-dl to youtube-dl-x. Though so far without much success due to lack of python knowlede.

So I guess we have to wait until the mytube developer or some other Python god updates the script.