From one moment on the other. Except 2 which I needed to readd recently.
This is becoming annoying. I need to readd these sensors every x days/weeks.
Changed channels already etc etc…
Have you got any repeating devices in your mesh, if so what kind? Have you looked at the map of your mesh?
First most of the devices were gone (in Phoscon). Now… 2 days later. Most of them returned. Some with the same name and are also recognised again in hassio, some I need to readd as they are still not reachable in Deconz and got a general name in Phoscon.
I have one gledopto downlight which loses connection regularly.
Apart from my conbee2, I have 3 powered zigbee devices. (2 lights and 1 plug)
Ok, and how many battery powered devices are in the mesh? Could it be distance / lqi related? Have you looked at the map via VNC to verify the lqi and the way the mesh builds itself? Is the conbee stick nearby a usb3 device?
I think it is related to the xiaomi zigbee implementation.
Not distance related. Some are very closeby. Not related to usb3. Otherwise it wouldn’t work for 3 weeks.
Could you tell me more about : ‘to verify the lqi and the way the mesh builds itself?’
I have seen a post about this as well but it’s complete BS is my opinion. I have a hugh number of Xiaomi sensors connected to a Conbee (spread over 4 floors!) and they all work fine never had to re-add a single one.
Please share some info about your setup, how are you running deConz, what does your network map look like?
Looks like this… Devices are added again now…
Hal Boven is a light that is not always detected (gledopto downlight, comes and goes)
Are there any specific special tweaks you can do in this program? I put the channel on 25. That seems to be farrest away from all 2.4 wifi channels
Do you use a USB cable to connect the Conbee as advised? When they loose connection they never come back automatically? You re-add them how, completely remove and search? Do you keep them in the same location or move to the Conbee? Did you try to just open/close the door or shortly push the button?
Maybe @frits1980 can explain howto check the lqi.
Most of them do come back but now and then I need to remove them and add them again.
I indeed remove them. Wait a few minutes and readd them.
I keep them in the location that they need to be. I also noted indeed if they are not seen and I open the door that it comes back (which looks indeed like they don’t answer the heartbeats from the server?)
You ask a lot of questions and not that much answers 
There are two ways to check lqi. In the map your see the color of the “wires” green is good. Yellow is worse and red is terrible. There is also an option in the map software to toggle lqi view by numbers. It’s in the top menu somewhere. Then the software gives the number plotted on the “wires” between the devices. (if you toggle the option in the menu it sometimes takes a couple of seconds before it shows in the map)
Yeah… makes it quite a challenge helping you with the problem 
Ok, first you do not need to remove the sensors, simply press the button once and they should reconnect. If not you can just search for new sensors and press the button. That should do the trick.
Second, if you are not using a USB cable to connect the Conbee, use it as it’s adviced in the official instructions.
Third, I have no idea 
start with the USB cable
The USB cable is already in place It really didn’t work good without. I have an external harddisk on USB3 nearby which was causing trouble…
What I also wonder. In dconz you can select one channel. But in Phoscon I can select multiple. Which I tried now. 11, 25 and 26.
Hal Boven is the one who comes and goes…
Which channel to select depends on your (and your neighbours ) wifi settings. I use 25 and my wifi is set to channel 6.
Your mesh isn’t very strong as none of the repeaters have a proper connection to another repeater. Hal Boven doesn’t seem to be able to get a proper connection to the Staande lamp and is not able to connect to your Buiten lamp at all, which leaves a not very strong connection to the conbee. I guess that is why it comes and goes.
It’s channel mask not channel selection. It doesn’t do anything to improve communication.