First thoughts are, check your snapshot reserve, you might see unexpected behaviour if the snapshot reserve is set to 0%
Second thought, have you tried a different browser? This particular problem doesn't sound like a browser issue, but i have seen some odd things in System Manager that turn out to be browser specific.
Final thought, i'm running OnTap 8.2.2 and 8.2.4 with System Manager 3.1.2 and havn't seen this issue. Is this possibly an 8.2.3 bug?
It does let me set it up if I set a specific snapshot reserve percentage. However, I have done this a lot in the past with no snapshot reserve since I make sure to create the volume larger than the LUN(s) inside. I guess I need to read up on what a good percentage is for snap reserve and start using it.
I tried using a different broswer, but it won't let me. No matter what I set my default to, System manager launches in IE. (I'm using the installable version for 7-mode)