Update (June 2015): several people have pointed out in the comments that there seems to be a authorization problem somewhere in the Google Apps script. Unfortunately I don't have time to dig in and look for the issue. If you find the problem, please let me know so I can update this post!
When Twitter released their API v1.1 in September 2012, they removed the possibility to create RSS feeds from timelines, lists or search results, and thus rendered many retweeting services and manually set up workflows disfunctional. One can rant all about Twitter moving away from their roots of providing an open communication ecosystem and how those restrictions clearly serve the main purpose of getting people to twitter.com to show them ads, but here are the steps you need to take to get your retweeting back on track.
It has become a bit more complicated but no rocket surgery, as you will see.