I'm experiencing the exact same behavior. Everything was working fine, but now one of my SSISd is not allowing connections and in the status it shows "Enabled" while the other (working) SSID shows "Connected"
I've tried rebooting the WAP but that hasn't helped.
Then I tried disabling security from the SSID that wasn't working and I was able to connect. I re-enabled security and it worked. Makes no sense at all to me, but everything is back to proper working order now.
Very strange.