Spurious empty white text box overlays Windows desktop in Storyboard 2 in Slide view

Has anyone else experienced this?

I loaded Storyline 2 this morning  to find that in Slide view (only) an empty white text box now appears from the top left. A screen grab is attached.

I can change its size by dragging the corner but can't get rid of it. It's a text box because the cursor changes to an 'I' when over it. It happens in a new empty Storyline file as well as now appearing in all my existing files. If I Preview the SL file, the window disappears. It also disappears if I display the 'Form' view of a question but returns when I display the 'Slide' view.

I re-booted Windows 7 Professional (running in Parallels Desktop 10 on an iMac OS X Yosemite). It isn't happening in any other Windows program on this machine.

I have just downloaded Storyline 2 .exe, uninstalled my existing SL2 program and re-installed the new program. The problem is still there!

My MacBook Pro is running an identical  Win 7 Pro/Parallels Desktop 10 setup with no problem. I re-installed SL2 Update 6 on the MacBook and there's no sign of the unwanted window on that machine.

What the heck is it? What have I missed?

Thank you.