If it tells you that, it is not running fine, just because there is a curve doesn't mean it reflects reality...
What does it mean when I get this message? Since it had always done this to me but ultimately gave me measurements I assumed it was just a quirk.
Another potentially related issue on Macs is a workaround I stumbled on a while back to get the measurement process going. Steps below:
Hit Measure -> Next -> Next (again)
Change buffer from 32k to 64k and then back to 32k
Then hit Measure and continue as normal.
Someone in a thread maybe a year back posted this series of moves to get REW to work on his Mac and it worked for me as well. If I don't do this I don't get any levels.
I guess I'll have to try using XP in Boot Camp and see if it works for me.