Cisco Wireless :: AIR-AP1131AG-E-K9 / Getting PPTP VPN Through Access Point?

my customer has a very basic LAN+WLAN configuration:
 
- 1 switch (not Cisco)
- 1 DSL router which is the default gateway (not Cisco)
- 2 Wi-Fi Access Points (Cisco AIR-AP1131AG-E-K9)
 
If he tries to establish a PPTP VPN when he is connected to the wired LAN, it works but when he tries the same from the wireless LAN it doesn't work. He says he'd tested it with Windows Firewall disabled. Is there any configuration command that can enable/disable PPTP VPN traffic pass through a Wi-Fi connection? I've never seen something like this and I use PPTP VPNs over a Wi-Fi connection every day without any particular configuration on the access point.
 
Here's the APs config:
 
hostname ...!enable secret ...!aaa new-model!!aaa authentication login default local!aaa session-id common!!dot11 syslog!dot11 ssid ...   authentication open    authentication key-management wpa version 2   guest-mode   infrastructure-ssid   wpa-psk ascii ...!power inline negotiation prestandard source!!!bridge irb!!interface Dot11Radio0 no ip address no ip route-cache !
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