I have now set my 3 Deco, which still has the firmware version 1.4.9 Build 20200918 Rel. 74680, as the main Deco and removed the one with the corrupted firmware update from the network. The devices mentioned above also no longer work with the old firmware version.

I can reproduce that the problem arose after the update to version 1.4.10 and that it works properly via other WLANs but no longer via the Decos.

I would like to know the exact changelog details please. With the following superficial information it is not possible to understand what has been changed. 

"Bug fixes and new features:
1.Fixed the bug that parental control doesn't take effect.
2.Supported HomeCare Malicious Content whitelist function.
3.Fixed some other bugs."

What does "Fixed some other bugs" mean? 

I have a TP-Link Archer MR600 as my router. Don't get me wrong, I'm a fan of keeping my systems up to date, but not under these circumstances. 

After the update, I had to completely delete the previously working WLAN on all tablets and reconnect them in order to bring them back into the WLAN. A factory reset of the Chromecast and the Nano router did not help. The Chromecast now thinks the Wi-Fi password is wrong, it is 100% correct. We can no longer print either. 

Thank you for this.

  @David-TP 

Hello and thank you for your feedback.

Admittedly I understand your statement that it has nothing to do with your product that it no longer works with the old firmware.

The clear counterargument is that it started exactly on the day I changed to the latest firmware. Coincidence? No, I don't think so, because with a hotspot or an access point from another provider, everything works with the same setup.

Nevertheless, I followed your suggestions. All Decos are now on the old firmware. I have tried it with easy and complex WLAN passwords, also set up a completely new WLAN. Also with fast roaming and beamformng enabled and disabled. I have also tried a WLAN with only 2.4 GHz and only 5 Ghz, without success.

WLAN Guest makes no sense for us, as we a) Access a NAS in the LAN with the Chromecast b) Scan to folder with the printer and neither is possible via the WLAN Guest.

One point for me is that the Nano Router in client mode can only do WiFi 4 it 2.4 GHz. So again, what has really been changed in your new firmware update? I lack transparency here. The Chromecast that runs on both frequency bands in WiFi 5 speaks against this. The Chromecast only says "No connection" possible.

I do not believe in coincidences in this case. Firmware update installed, problem there, on several devices. No changes have been made to the Main router either. 

Thanks for your help and further suggestions.