Are one client or all client impacted to the AP in question before AP reboot.It appears 4-way handshake is broken. it could be AP or client related until proven.if its one time issue I'll move on, otherwise when issue reproduced involve cisco TAC to ...

are vlan6 is wireless management.Does vlan8 have SVI on WLC, is it configured to use VLAN8 as a source interface.

Try this -1.does 40mhz work as expected.2.on AP, manually set ch to 36, get client close to AP and test it. now capture rssi/rate/index.3. if possible take over the air packet capture to see what's going on.