Not able to run reports for copied or imported backed up projects

Why do I get the message "This report was created for another application or a previous version, and contaings references to datea fields which no longer exist.  To fix this report, click on the Modify botton to edit in the Report Editor."  This happens everytime on the same project copied or imported after exporting it.  Tried exporting it as XER and XML.  I've tried the Work Around suggested by Knowledge Base of adding a blank row in the Report Editor and it does help.  FYI, I'm using P6 7.1 stand-alone and activity codes are under Project Codes and not EPS