We are using WLC-5508 in our corporate. For authenication we have implemented ACS with LDAP configured as external user database. We can able to get authenicated for Web based authenication. When it is configured for EAP-FAST, authenitication is not happening.
If you check the above link that i have shared with you EAP - FAST phase 0 is not supported when LDAP is the external DB integrated with ACS. If you try with another EAP type such as EAP GTC or EAP TLS it will work for sure.