Yes, I turned on user debugging on the phone. I use WPA2-PSK/AES for encryption, but the phone doesn't have problem with encryption. The messagges of deauthentication are these:
Oct 29 10:44:46 stm[2310]: <501105> <NOTI> |stm| Deauth from sta: 00:03:2a:25:1b:4a: AP 10.20.0.39-00:24:6c:80:ae:b4-APADU Reason Tx fail reached maximum
Oct 29 10:44:46 authmgr[4251]: <522036> <INFO> |authmgr| MAC=00:03:2a:25:1b:4a Station DN: BSSID=00:24:6c:80:ae:b4 ESSID=WiFi_RED VLAN=3 AP-name=
Oct 29 10:44:46 authmgr[4251]: <522234> <DBUG> |authmgr| Setting idle timer for user 00:03:2a:25:1b:4a to 15300 seconds (idle timeout: 15300 ageout: 0).
Oct 29 10:44:46 stm[2310]: <501000> <DBUG> |stm| Station 00:03:2a:25:1b:4a: Clearing state
Oct 29 10:44:46 stm[685]: <501106> <NOTI> |AP APADU@10.20.0.39 stm| Deauth to sta: 00:03:2a:25:1b:4a: Ageout AP 10.20.0.39-00:24:6c:80:ae:b4-APADU handle_sapcp
Oct 29 10:44:46 stm[685]: <501080> <NOTI> |AP APADU@10.20.0.39 stm| Deauth to sta: 00:03:2a:25:1b:4a: Ageout AP 10.20.0.39-00:24:6c:80:ae:b4-APADU Tx fail reached maximum
Oct 29 10:44:46 stm[685]: <501000> <DBUG> |AP APADU@10.20.0.39 stm| Station 00:03:2a:25:1b:4a: Clearing state
Oct 29 10:45:49 stm[685]: <501106> <NOTI> |AP APADU@10.20.0.39 stm| Deauth to sta: 00:03:2a:25:1b:4a: Ageout AP 10.20.0.39-00:24:6c:80:ae:b4-APADU handle_sapcp
Oct 29 10:45:49 stm[2310]: <501105> <NOTI> |stm| Deauth from sta: 00:03:2a:25:1b:4a: AP 10.20.0.39-00:24:6c:80:ae:b4-APADU Reason Tx fail reached maximum
Oct 29 10:45:49 stm[2310]: <501044> <NOTI> |stm| Station 00:03:2a:25:1b:4a: No authentication found trying to de-authenticate to BSSID 00:24:6c:80:ae:b4 on AP APADU
Oct 29 10:45:49 stm[685]: <501080> <NOTI> |AP APADU@10.20.0.39 stm| Deauth to sta: 00:03:2a:25:1b:4a: Ageout AP 10.20.0.39-00:24:6c:80:ae:b4-APADU Tx fail reached maximum
Oct 29 10:46:51 stm[685]: <501106> <NOTI> |AP APADU@10.20.0.39 stm| Deauth to sta: 00:03:2a:25:1b:4a: Ageout AP 10.20.0.39-00:24:6c:80:ae:b4-APADU handle_sapcp
Oct 29 10:46:51 stm[2310]: <501105> <NOTI> |stm| Deauth from sta: 00:03:2a:25:1b:4a: AP 10.20.0.39-00:24:6c:80:ae:b4-APADU Reason Tx fail reached maximum
Oct 29 10:46:51 stm[2310]: <501044> <NOTI> |stm| Station 00:03:2a:25:1b:4a: No authentication found trying to de-authenticate to BSSID 00:24:6c:80:ae:b4 on AP APADU
Oct 29 10:46:51 stm[685]: <501080> <NOTI> |AP APADU@10.20.0.39 stm| Deauth to sta: 00:03:2a:25:1b:4a: Ageout AP 10.20.0.39-00:24:6c:80:ae:b4-APADU Tx fail reached maximum
This happens randomly; today for example it happens continuosly, but for the three days ago It was never happened.
Others WIFi Phone (e.g. Snom or Polycom) connected to the same SSID don't show any problem.