After upgrading one of my two Zeroshell Network Servers from 1.0.beta9 to 1.0.beta10, the problem started.
The Captive Portal on 1.0.beta10 was forwarding all Authentication to the Captive Portal on 1.0.beta9. Once I stopped the Captive Portal on 1.0.beta10 from forwarding its Authentications the problem stopped.
Even through it is working, I am still unsure of the problem’s origin because the Captive Portal was assigned to the Guest Wireless Port and has not been in use for several weeks.
I do not believe that the issue was an incompatibility issue between the two versions. It just happened to be the only solution that worked for me, because of my limited knowledge and experience.