Good day,

I have 2 hap ac, running currently 6.38.1, one access point runs capsman for both, 802.11ac cards are set up with a local forwarding

The clients (Intel AC 7260) work without any problems with 80Mhz/2S/SGI when running the configuration without capsman, with capsman of 6.38.1 SGI is unavailable.

So I've tried to install 6.39rc20 today, the clients connect without any problems having SGI enabled however after transmitting some amount of data they disconnect with "extensive data loss" in routeros logs.

please provide us with more detailed info on the error that you have and the setup.
They disconnect and reconnect back?
What amount of traffic is transmitted when the problem happen?
Does it happens also when using the SGI without CAPsMAN?

please provide us with more detailed info on the error that you have and the setup.
They disconnect and reconnect back?

sometimes it reconnects after a delay, sometimes doesn't until reconnecting manually

What amount of traffic is transmitted when the problem happen?

well, pretty quickly. it's enough to upload ~500MB+ file and that's it.

Does it happens also when using the SGI without CAPsMAN?

Nope. I've run these hap ac w/o capsman for a month, it never happened. The problem also disappears when I turn off capsman for wlan2 card and set up 802.11ac locally. All settings are default except I specify a frequency and channel extension (eeeC), same as in capsman of course.