After months of testing I'd say that most dropouts occur when I download or (even more often) upload a large file (hundreds of megabytes or a couple of gigabytes). So I did some more googling and...

@terziyski Unfortunately I never manage to get to the router quickly enough to look at the LED lights. The router is located in a different part of my house than where my computer is and the dropouts...

@terziyski Yes, I will stick to the C6 v2 at the moment and we'll see what the next stable firmware version brings. I do have WiFi all over my house but that is managed by several Uniquiti Unifi PoE...

@terziyski Thanks, sadly it did not help. I upgraded to the beta FW and I disabled OneMesh. For the whole day it seemed that this might have helped... but in the evening came the first 15-second...

I've always been recommended TP Link as a good brand for home routers so I bought C6. I use it only for wired connection (for my home network) but I'm experiencing very frequent short dropouts....