The evaluation version apparently has two modes, Evaluation and Player. I believe in Player mode you will have trouble loading your 5.1 vi's. Look :
here for details on how to figure out if you're in Player mode.
LV 7 has opened my LV 5.0 and 5.1 vi's with no error. If you want to post a 5.1 vi (not too big) to see if they open properly, I'll give it a try.
I recommend upgrading - lots of nice features in the latest revs.
Tim