Snom D865 Wlan problem

Setup:
- On client side:
- D865 connecting with Wifi by the Wlan on the Phone
- D865 connecting by Direct stun to the 3cx V18 PABX

Result:
- no problems. Phone is connecting and is in the Phone list of the 3cx admin console

Problems:
- After more than 48 hours the user is disconnected on the phone
- It is still possible to go locally to the webinterface of the Phone so there is connection over Wlan
- On the same time this is happening we get following screen



Investigation done:
- Reboot is resolving the problem and it comes back after more than 48 hours
- Same setup on a other wifi network and location same problem
- 3CX has other snome phones (different model) that works with direct stun without problems
- Snom is on the latest Firmware version by 3cx: snomD865-SIP 10.1.169.13
- Conneting phone over cable POE no problems even after 5 days

Question:
- What is the Wifi logo with question mark means? I supose problems with the wifi although i can connect to the webinterface on the same time that the Line from the user gives a red cross and is disconnected.
- Any advice ?