We found the same issue and reported it to Ruckus (along with a LOT of other issues with vSZ). They have so far not resolved it, but said it is probably related to them over-reporting connection failures. For us, if you go to the Dashboard and Connection Failures, we see 70-85% failure rate overall, with the same for the first category of "Authentication". The other categories: Association, EAP, Radius, DHCP, are all below 1% almost all the time. We have 2500 clients, and they all are connecting fine now. If 80% of them weren't connecting, I think someone might tell me.
So Ruckus is mis-reporting, or mis-categorizing something. The definition of "Authentication" from the Dashboard is: Authentication failure is a measurement of client connection attempts that failed at the 802.11 open authentication stage. This is the first stage in any modern Wi-Fi connection.
My guess is that our clients are roaming and start a connection, but don't complete it with that AP, but instead have moved on before it can complete the connection. But that still seems like it can't explain all of this, because wifi should connect fairly quickly, and our people walk (slowly, especially when I'm behind them in the hallway) around, and really shouldn't be passing to many APs without connecting to them before moving to another AP.