I remember seeing this a while back when I was doing something wrong! I don’t remember exactly what it was, but I think it was a setting in the Constellation Visualizer, right-click and go to Component Properties. It might not have the proper bit rate or symbol rate, or the constellation type is not correct.
Try making sure all those settings are correct. I tend to always trust the results of the BER Test Set since it is a fairly simple component, so the error must be somewhere else.
Cheers!