Yes, indeed.
We also configured a DHCP on our firewall for this SSID for testing purposes and disabled the DHCP on the third party server. When the third party server was NOT connected, we were able to get IP address from DHCP on our firewall and we could ping around in the network. But when we reconnected the server to the network, ping stopped and we could not get IP anymore to wireless clients.
Then again when we disconnected the server, the ping continued instantly...
We then reconnected again the third party server (obviously the continuous ping stopped at this moment) and monitored the switch port where it was connected with Wireshark and noticed that lots of multicast packets (protocol MPEG TS) was sent from the server. Then enabling IGMP on the VLAN on the switch where the server was connected and instantly ping went through. And now we moved back to original plan using the third party server as DHCP and it worked also...
So there, don't know why this happened but maybe someone has good theories?
Since wired clients could get IP address all the time, but wireless clients could not, I was thinking maybe enabling IGMP on the Aruba controller would also have fixed this?