4 years later and still no solution.

No, I do not have that configured. I do not know which key I would have to configure. Provider Orange did not give any user/pass or key to me. The interesting part is, that the Cisco router accepts the DHCP-provided IP as soon as it failed once with ...

@mathieupoussin  had the problem on a 1941 router IOS15. I have the problem @mathieupoussin  described on a 1841 IOS 15.1(4)M12a.  interface FastEthernet0/1 no ip address duplex auto speed auto no cdp enable ! interface FastEthernet0/1.832 encap...

I have the same problem and found the exact same behaviour.  Did you ever solve the DHCP problem with Orange in Spain?

Locally switched guest and central web authentication works. Make sure the "Local Net Users" are set up for the right WLAN, or chose "Any WLAN".