[LAU] Can't save files in Qtractor

I have a desktop and a laptop computer, both running Ubuntu 18.04, and both with Qtractor installed. On the desktop computer, when I start a new project in Qtractor and click on "Save As", the expected dialog appears. As expected, the "OK" button is greyed out until I start to enter a name for the new project. At that point the "OK" button becomes usable, and the project can eventually be saved. However, on my laptop computer, the "OK" button remains greyed out all the time. This is true whether or not I have made any changes since starting the new session. So I can't save any of my work. Currently, therefore, Qtractor is effectively unusable on the laptop. What could be causing this problem? David