Yes, downloading the Bela-master branch .zip file and updating it was the first thing I did. But that gave me the errors when trying to import the lecture code examples. So I updated the Bela image as well.
Anyway, I solved the problem meanwhile, as unzipping the sine-generator examples on my own Ubuntu laptop gave me the same error. I just downloaded the example zip files in the wrong way: I went to the code examples folder on Github, right-clicked on the files and then on Save Link As. This will present a zip-file to download, but it's actually just another webpage. The right way to download the file, is to left-click on the files, which will lead to another page, where clicking on Download will give the actual file.
They're all working fine now. Thanks for this interesting course!