AP55 are going offline randomly on SG 135W (9.705)

Hi all,

I discussed this over 3 years before here (https://community.sophos.com/sophoswireless/f/discussions/104290/ap55-are-going-offline-connected-to-sg-135w-9-509-3) and it is coming back now.

Sophos SG135W, AP55 as mesh root, 3 AP55 as mesh AP. The mesh APs are going offline randomly. Actually 1 or 2 times a day. After times (about (20 min to 1,5h) there are coming back without any doing. The SG itself is also part of the wlan.

First thing I wonder:

I expect, that clients cannot connect to the mesh root. But I am wrong: The mesh root has the most average client load and highest maximum client connection load from all.

Second thing:

the router itself always shows connections in wireless lan overview but shows zero connections in "Logging and Reporting" -> wireless. So what is right?

Third thing:

When Sophos send the "AP is down mail" absolutely no info is shown in the wlan logfile.

Any ideas?