"Can't reach the server" ("Ik kan de server niet bereiken) after installing new FRITZ!box 7581 WiFi

I really enjoy working with my Homey 2.0.4 (Early 2016). Thanks to the great developers I created a lot of cool flows that works seamlessly.

My provider offered me a month ago a faster connection and a new Wifi-router, a FRITZ!box 7581 with dual WLAN. From that moment on Homey looses from time to time connection, resulting in the message: ‘Sorry, I can’t reach the server. Try again later.’ Sometimes for hours it works perfectly and then the problem appears. After trying 2, 3 or 4 times trying it works perfectly again.

I tried many things:
-installing the old FRITZ!box (3490) again (didn’t connect to the internet)
-removed the repeater (from FRITZ!box)
-on the FRITZ!box put the profile of the Homey on ‘unrestricted’
-enabled options
-‘assign Homey to the same IP4v address’
-‘allow access for applications’
-‘send status information via UPnP’
-‘setting up FRITZ!box as a time server in the home network’

The strange thing happened when I connected Homey to the Guest section of the network of the same FRITZ!box (7581). Now it works perfectly without any loss with the server. Unfortunately Homey isn’t able to share data with other WiFi devices like the Harmony hub, Evohome, et cetera, so it is no solution for me.

Can anyone help me?

This sounds like you are using the Band Steering that comes with the router and the Homey has issues with that. In the band steering mode you have only one SSID Network that uses both the 2.4GHz and the 5 GHz wifi combined and the fritzbox determine what device using what type.
As homey is only capable of using the 2.4GHz Wifi Network, this can lead to issues. The normal guest network don’t use this technik in the factory default setting and only provide an additional 2.4 GHz Wifi that you Homey is normal working.
You can get rid of Band Steering, if you dedicate two different SSIDs for your networks. But then there are other issues.
You can find some informations to that on the AVM Homepage (German Link!):

It should also be possible to assign your Homey to the 2.4GHz wifi, but i’m currently unable to provide a info how to do it.

Hope this will help

Also look at @johan_bendz 's post about wifi here

Thank you for responding this fast! I really appreciate it.

The FRITZ!box creates 2 separate networks: a 2,4 Ghz and a 5 Ghz network with both another MAC-address. It always connects to the 2,4 Ghz, also tried to connect to the 5 Ghz, but like you said Homey isn’t capable, so it refused connecting.
But it seemed that both networks don’t interfere with each other, or am I wrong?