1. The file is compromised. Solution: use a backup copy.
2. Quattro Pro is compromised. Solution: reinstall
The most likely is #1. So let's hope you have a recent backup.
I have a Quattro Pro 11 spread sheet that I have opened many times. When I opened it yesterday a small rectangular box with no writing in it the size of a reduced file box appears in the bottom left corner of the spreadsheet. None of the tabs on the spread seet can be accessed. This locks up the spreadsheet and makes it unusable. When I right click on the box I get a drop down menue of: restore, move, size, min., max, close, etc. When I right click on the box I get the "Active Notebook" popup menue. I have been using Quattro Pro and WordPerfect Office for years and never seen anything like this. Does anybody have a solution?