That happens when opening a project that was saved with a different QT version. The issue comes from the .pro.user file. You can remove it (while your project is closed in QT Creator, so it won't be recreated automatically) and then reopen the project. The bad thing is that you'll have to reconfigure the project options: Targets, build settings, editor settings...
Maybe it could be done editing the old .pro.user file and change/remove the proper fields but it doesn't seem an easy task.