@kluszon 

Hey

Welcome to the world of 2.4ghz..

Sorry but what you have described is fairly common and likely not an AP issue, rather a limitation of the frequency used (2.4).  

In short the older 2.4ghz is pretty much saturated in almost all cases, there is a few things you can do but honestly.. 10 devices is really going to strain airspace for 2.4ghz

Try these first and see how you get on

1.  On the AP enable AirTime Fairness.  This will help negate any devices hogging the airspace, its a good idea for IOT devices and generally in most installations I would enable this on 2.4 by default (leave off on 5ghz)

2.  Check the channels in your area.  If you neighbours are on 1 try staying to 11, sadly its likely all channels have some interference already

3.  Keep at 20mhz channels, more capacity at the expense of speed

4.   Move as many devices onto 5ghz as possible..

In short, 2.4 is a pain!   you will see dozens of posts on the forums with people complaining the same issues you are having.

  @kluszon are you in control of the clients? If so disconnect all that you can and add back one at a time. See if the problem goes away with less clients. If it does start adding until you see the issue show up.  That will at least be a starting point.

My network consists of 4 EAP225 and 1 EAP610 wired. We have an average of 30 clients. 20 of those are 2.4 and we have no reported issues, but there are so many factors that you can't use one persons results as a base. My first impression would be interference from something.

Wish I could be of more help but start with process of adding one device at a time.

  @Philbert 

1.  Enabled AirTime Fairness didn't change anything in this case.

2. As I wrote before the location is a private house and I don't have any 2.4GHz networks around.

3. I set the channel width to 20MHz without any change for stability.

4. I didn't solve the 2.4GHz network problems.

For me, it's strange because after the factory reset AP is working perfectly for 3 hours with all devices on 2.4GHz, but after this time the problem is back. Moreover, I have EAP225 with hardware version 3 and there everything is working perfectly also on 2.4GHz!

  @kluszon 

Ah one of those random device plugged in that interferes with everything! 

It was likely just forcing the retry and error rate high enough to kill the 2.4ghz, its a frequency known to not be forgiving..   Good find all the same and glad its working for you!