Issues with a second SSID on Fritzbox 4040 / IPQ4018

Hello all,

i have issues with a second wifi SSID on 2.4Ghz on my Fritzbox 4040.

The router is connected to the device I got from my provider via WAN and I have two wifis configured. One on 2.4 Ghz which I use for my laptops. Another on 5Ghz which acts as a bridge to another router (TP LInk Archer C6). So far so good. This setup works for almost two weeks now.

Today I wanted to add another wifi on 2.4 Ghz which means two different SSID on the same radio. I created it via LuCi. First added it on the wireless tab, then configured the interface as static IP and activated DHCP. Also created a new firewall zone for it.
Now when I try to connect to it it takes quiet a long time then my laptop and smartphone say it's not possible and switch back to their "old" wifi.

In the system log I get the following messages. The first line is my laptop disconnecting from the first ssid on 2.4Ghz (wlan0). The last line is the laptop connecting back to the first ssid (wlan0).

Mon May  1 09:55:43 2023 daemon.notice hostapd: wlan0: AP-STA-DISCONNECTED 68:a3:c4:78:69:80
Mon May  1 09:55:43 2023 kern.info kernel: [672643.081473] ath10k_ahb a000000.wifi: mac flush vdev 0 drop 0 queues 0x1 ar->paused: 0x0  arvif->paused: 0x0
Mon May  1 09:55:43 2023 kern.info kernel: [672643.350583] ath10k_ahb a000000.wifi: mac flush vdev 1 drop 0 queues 0x2 ar->paused: 0x0  arvif->paused: 0x0
Mon May  1 09:55:45 2023 kern.info kernel: [672644.611101] ath10k_ahb a000000.wifi: mac flush vdev 1 drop 0 queues 0x2 ar->paused: 0x0  arvif->paused: 0x0
Mon May  1 09:55:46 2023 kern.info kernel: [672646.270444] ath10k_ahb a000000.wifi: mac flush vdev 1 drop 0 queues 0x2 ar->paused: 0x0  arvif->paused: 0x0
Mon May  1 09:55:48 2023 kern.info kernel: [672648.471048] ath10k_ahb a000000.wifi: mac flush vdev 1 drop 0 queues 0x2 ar->paused: 0x0  arvif->paused: 0x0
Mon May  1 09:55:59 2023 kern.info kernel: [672658.630959] ath10k_ahb a000000.wifi: mac flush vdev 1 drop 0 queues 0x2 ar->paused: 0x0  arvif->paused: 0x0
Mon May  1 09:56:12 2023 daemon.info hostapd: wlan0: STA 68:a3:c4:78:69:80 IEEE 802.11: associated (aid 1)
Mon May  1 09:56:12 2023 daemon.notice hostapd: wlan0-1: Prune association for 68:a3:c4:78:69:80
Mon May  1 09:56:12 2023 daemon.notice hostapd: wlan0: AP-STA-CONNECTED 68:a3:c4:78:69:80

I already tried googling these messages but didn't find anything. Do you have any hint how to solve this?

Thanks in advance
Kevin