As we try to keep legacy devices away, we use minimum bitrate 12Mbps, my networks are actually high density networks, so it is easier to handle roaming under this set of bitrate, however, I've tried getting it up and down as test, didn't worked.
I don't see this as an RF/data rate/layer2 issue but you can easily rule it out if you create a test SSID with PSK authentication. If your device stays connected you're good on that front. The log you posted clearly shows the disassociation is occurring after multiple 'not authenticated' log entries. So your device is associating successfully to the WLAN but is unable to pass traffic due to its 'not authenticated' status.
Have you looked at the RADIUS server logs yet though? I get the feeling this will tell you most of what you need to know. If you were able to post the session log it would give a better idea what is going on.