First, with Legacy closed, open your Documents folder and browse into the \Legacy Family Tree\_AppData\Usr sub-directory. Next, delete the RptProbs.usr and ProbsList.usr files. They will be recreated when you go back to the Potential Problems Report.
If that doesn't help, go back to your Documents, and then go into the \Legacy Family Tree\Data sub-directory (or whichever folder you have your Family File stored in) and delete the .PPL file (it has the same "first name" as your Family File).
Try your Potential Problems report again. It should work.