I have the exact same problem. It happens every time either one of our Radius servers crashes or is rebooted.
The Radius server crashes, or goes down/reboots, and the captive portal service hangs.
I need to fix this problem as we cannot do without our captive portal and having to reboot our firewall every time one of our Radius servers goes down is disruptive to our business.
Even a command-line fix that did not require a reboot would be extremely helpful as we could automate that through Nagios.
Edit: Actually, to clarify, I don't know whether I'm getting the same "errors" the OP is getting as I am unsure of how to diagnose this issue, however the symptoms I have are the same.