@MichaelArn 

Start with checking the quality of WiFi backhaul between Satellite Deco(s) and Main Deco. You can find that info in Deco app, example:

In this example Satellite Deco Bedroom has acceptable quality of link (two green bars) to Main Deco.

If yours is all good, please post screenshot of that page here, and also Show Map page, and provide the following info:

1. What is the speed of your Internet connection;

2. What ISP modem/router do you use: brand and model name;

3. Do you run your ISP modem/router in modem or in router mode;

4. Do you run Deco mesh in Router or in Access Point mode.

  @Alexandre. 

Thanks for the quick reply!
The connection to the satellites should be okay.
It's about the satellite "Wohnzimmer". Here are pictures (i´m from Germany, therefore it´s in German):

1. The speed should also be fine, the download speed is between 85-100 mbits

2. I use a Vodafone Easybox 804 as modem/router 

3. I'm not sure how you mean, the Easybox is connected by cable to the DSL connection and the main deco is connected by cable to the Easybox. The Easybox is now no longer active since using the Deco, I also turned off the WLAN

4. The Deco runs in Router Mode

Kind regards!

  @MichaelArn 

WiFi backhaul link appears to be OK. You can also check under Satellite Deco page in the App if connection to that Satellite by devices is good, and I am going to assume it is.

Another possibility is your Main Deco can't handle the load. The way to reduce load on Main Deco is to configure Deco mesh to run in Access Point mode. In that mode Main Deco will no longer apply content management rules to passing traffic (which taxes Main Deco CPU) and will just pass traffic to/from Vodafone Easybox.

That will only work if your Vodafone Easybox 804 runs as a router. It is not clear from your reply if it is: you said "Main Deco is connected to Easybox" but also "The Easybox is now no longer active." I can't figure what does it mean in regard to Easybox, without additional explanation.

If Vodafone Easybox runs as a router, you can configure Deco mesh to run in Access Point mode by following this document: How to set up Deco to work in Access Point mode

  @Alexandre. 

You're right, I worded that badly. The Easybox is of course active, it has to supply the Deco. It is connected to power and DSL connection. It is connected to the main deco via a LAN cable. But the Easybox doesn't deliver direct WLAN anymore, I switched off the WLAN of the Easybox. Does that still mean that the Easybox runs as a router and I can try that with the access point mode?

  @MichaelArn 

I start thinking that your Easybox does not run as a router, otherwise Xbox would have complained of "Double NAT detected."

This is what you can do:

Change Easybox to its original configuration, the way it were set by your ISP. Everything as it were originally, before Deco, but keep Easybox WiFi off.

After you've done that, verify there is still access to the Internet through Deco mesh. There should be. Your Xbox may start complaining of "Double NAT detected" and VPN client might not like it either, but that will be fixed in the next step. As long as LED on Deco shows it is OK and Internet browsing works on smartphone/tablet/PC connected to Deco mesh, all is good.

Now, the next step: switch Deco mesh to Access Point mode, following document. After that, Xbox will be happy again and VPN client should work, as you now back to a single router, which is Easybox.

If you got successfully to that point, now it would be time to test if disconnects went away.

  @Alexandre. 
 

Thank you very much!

I have succesfully completed all the steps and will now test and observe if the problems are resolved. I will report again.

  @Alexandre. 

I have now tested the connection with different devices.

Both streaming and Xbox work fine.
The only problem is still when I want to work remote with the laptop and connected to the satellite Deco. This is really strange and I can't figure it out.
When I am connected via remote desktop, I only have about 9 mbits in download and upload according to Speedtest. This doesn't seem to be a client problem though, as I get nearly 100mbits locally. And that's always the case, so both when I'm connected to the main deco and also to the satellite deco the speedtest gives the same results. But: when I am connected to the main deco, I can work without problems. But when I am connected to the satellite deco, there are constant problems.
Anyway, I have now additionally activated the WLAN of the Easybox as a test. When I connect my laptop to it, I have the same download speeds remotely (so this seems to be company related), but the connection is stable.
Very strange, but I can live with this solution, the WLAN of the Easybox and the WLAN of the Deco do not seem to interfere with each other.
Thanks again for your help!