BUG: Many Wireless Clients can't connect to AP15 anymore

Hi,

after installing the current Beta Build, some of my WLAN Clients can't connect to my AP15 anymore.
I've got 2 Raumfeld Connectors and one Fritz Repeater 1750E that can't connect to my AP15 anymore. (They can connect to other access Points of other vendors.

My Tablets an Smartphones can still connect to the WLAN of the AP15.

Any ideas?