R7500: Sudden drop in 2.4 G download speed, upload is still good

In addition: I reinstalled the latest firmware (same version as before) and afterwards made a factory reset (reset button for about 10 sec pressed). 

Test right after this procedure did not show any improvement.

Reinstalled my own settings.

Interestingly enough now the R7500 shows up twice in my connected devices in my router: Once with the MAC address that have always seen so far (and that the router knows) and one more time with a MAC address that is next in ascending order to the original MAC address.

I tried multiple resets at router and R7500AP - no improvements.

Thanks.

WPA2 and AES always was selected.

I tried the channels you proposed, but nothing improved.

Currently i left it at ch 11 (other APs under same SSID are at ch6 and ch1). Before all were set to automatic channel selection, but spacing between them was always good.

I used 2 devices to check the speed (in addition to noting the slow speed in regular usage), both with three different software apps. Also verified at other access points that the readings make sense. Therefore I assume that there is no wrong analysis.