[LAU] Can't save files in Qtractor

[David Sumbler]

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