@Philbert, You nailed it. That was precisely the issue. Thanks a lot. All the problems with my Wifi Smart Home devices that constantly lost connection are gone. So the rule is: Never mix the 2xx series with the 6xx series?

  @aravaca 

No I wouldnt say that at all, mixing hardware isnt an issue as long as they are all using the same security cipher.    As long as you remember you can only go as new as your oldest device will allow.

Eventually there will be a stage where the gap is too wide and you will have to replace older hardware, but as both APs support WPA2 AES you are good for now.    WPA2 AES isnt likely to be depreciated for a few more years yet! 

  @Philbert 

Thank you for your advice. Nevertheless, I receive a security warning when connecting to my Wi-Fi access point.  It looks like it connects with the lowest WPA security layer. So, no WPA2 and no WPA3. I'm a little frustrated that the hardware I bought just eighteen months ago seems to be already pretty much outdated  

Does it make sense to replace the 615 with a 235 that I have still in stock so that all four access points are the same model? Can I then switch to WPA2/3?

  @aravaca 

You should only be getting the warning of security if you are NOT running AES encryption.  Basically WPA2 AES or above should not display this, can you post a screenshot of your security cypher?

In terms of hardware, yeah it would be easier to admin for you if you had all 245s (or even a 225 would do).    However this should be avoidable by cypher selection, a screenshot would be excellent if you can provide one.

What version of firmware have you on your OC200?

  @Philbert 

I already changed the setup to 4 x 235 access points, WPA 2/3, and have no issues. I can connect to any AP, and there are no security warnings on Apple devices anymore - the setup and quality I always expected.

Why a mixed setup of access points leads to such troubles with hardware bought in 2022 remains. I don't have the feeling it is excellent engineering. 

  @aravaca 

Its not engineering, rather a client side issue and Apple being Apple.

Basically the Apple device on first connection to the SSID will look to see what is the maximum strongest cypher it can use.   If its connected to the 6xx AP it will opt for WPA3 SAE and connect.

Now it moves to a 245 AP and tries to connect using the WPA3 SAE, it fails as not supported on the AP.      If the iphone had of connected first time to a 245 AP using WPA2, it would work on the newer APs also.

Its only apple tends to do this, ideally try not to mix hardware that supports different ciphers to avoid it.   I have Windows and Android devices in the same setup you have mixing 6xx with 2xx APs and it doesnt cause an issue. 

If you had all APs on WPA3 (6xx range) again this wouldnt be an issue