To download data from the website, click the name of the device where the data is stored. Opening a device and/or folder may take a few minutes, especially if it contains a large amount of data. When you find the file or folder you wish to download, click the Download button and choose where to save the file on your computer.
The web interface is fine for convenient access to individual files that aren't too large. But for big files or folders, or for many files, any web-based solution is going to unpredictably time out or unpredictably serve a broken zip archive. For reliable access to big files or folders, the SpiderOak ONE application on your computer will give better results.
True, sometimes you may not have convenient access to the application, but in those cases you are likely going to need only one or a few files. If you need more than that, then you will find it worth the couple of minutes it will take to install the SpiderOak ONE application. HTTPS was not designed to reliably serve more than webpage-sized files.
Alternatively, you can open the web interface, copy the link to the desired file or folder, then pass that link to your preferred download manager.