If 802.11ac-enabled clients are unable to connect with a LANCOM access point that is using LCOS 9.10RU3 or lower with its radio settings in the 5-GHz 802.11ac-only mode, this is because the clients are unable to handle the extended supported rates IE in the beacons.
This information element is only included in the beacons if the access point is operating solely in 802.1n (greenfield mode) or solely in 802.11ac mode. In the a/n/ac mixed mode, everything works as expected.
So far, iOS clients are not affected.
If you need to operate faulty clients of this kind, the problem is avoided by updating LCOS to a Version as of LCOS 9.10RU5 or higher.