Hi Rene,
Thanks for confirming, so we now know it's NOT an AP-related issue.
I'm not an ICX expert unfortuanately and have only just installed my own, so please bear with me! The easiest way is perhaps via the HTTP/S web interface. Navigate to CONFIGURE>PORT>ETHERNET this shows the configuration status of all your switch ports. Is there anything here that looks different on the port you're having problems with compared to the ports that are working ok?
If that doesn't work then in the switch CLI type 'show run' - this will show the full configuration of your switch and will certainly show if you have a configuration issue with your VLANs. I would guess that you've perhaps not got all the required VLANs configured on the switch port that uplinks to the rest of your network.
Thanks,
Darrel.