MikroTik

Have a RB411GL access point running nv2 in the 5GHz band, with 2 clients connecting to it. It is set to 5GHz/A with a 20MHz channel width, and so are the clients. No problems for a very long time. Then added a third client and set it to 5GHz/A/N and 20/40 Ce. After this one of the initial client stations started to have connection problems. From time to time it disconnects completely, and stays disconnected for maybe an hour or so. AP and the two ok clients are running ROS 6.42.6. Problem client is running 6.43.8.

The log on the AP says: "disconnected, control frame error". On the problem client the log has many entries indicating connection problems. After a while, however, the connection is established and seems stable for a day or so...
Any ideas what is going on? Bad client hardware, new noise in environment, ROS version "mismatch"??

Parts of the client log:
# feb/12/2019 12:59:47 by RouterOS 6.43.8
10:58:44 wireless,info 00:0C:42:6A:F0:58@wlan1: lost connection, control frame timeout
10:58:55 wireless,info 00:0C:42:6A:F0:58@wlan1: failed to connect, on 5240000, ranging timeout
10:59:12 wireless,info 00:0C:42:6A:F0:58@wlan1: failed to connect, on 5240000, ranging timeout
10:59:44 wireless,info 00:0C:42:6A:F0:58@wlan1: failed to connect, on 5240000, registration timeout
11:00:01 wireless,info 00:0C:42:6A:F0:58@wlan1: failed to connect, on 5240000, ranging timeout
11:00:40 wireless,info 00:0C:42:6A:F0:58@wlan1: failed to connect, on 5240000, registration timeout
11:01:21 wireless,info 00:0C:42:6A:F0:58@wlan1: failed to connect, on 5240000, registration timeout
11:02:08 wireless,info 00:0C:42:6A:F0:58@wlan1: failed to connect, on 5240000, registration timeout
11:02:49 wireless,info 00:0C:42:6A:F0:58@wlan1: failed to connect, on 5240000, registration timeout

-Olaf-