I just reinstalled the firmware via bootloader, set up the wifi connection and started the AP. Directly after the first reboot, the router did not connect to my wifi network. The kernel logs are the same as in my first post.
What i noticed is that when the mentioned error occurs and i deactivate the AP, what wireless module restarts (according to the status information in the upper right corner) but this restart does not cause the wifi client of the router to reconnect. Even if I restart the whole router this will not happen. Only when I deactivate and reactivate the wireless station in the WLAN menu, the router connects to the external wifi.