It seems that the router receives some invalid information from xx:xx:xx:6e:4f:15 device and initiates TKIP (Temporal Key Integrity Protocol) countermeasures due to MIC (Message Integrity Check), which then disconnect all wireless devices.
It may not be related to that specific device, but rather a number of clients and overcrowded spectrum.
Could you try the following:
Login to router's WebUI, navigate to Network -> Wireless. Edit your RUT955 access point interface's settings, switch to Wireless security tab and change Cipher option to Force CCMP (AES). See if that does not conflict with other devices capabilities and help to mitigate the issue.