@Zarquon 

The error message you see could be Deco running out of IP addresses to allocate to devices. If this what really happens, symptoms you described and temporary fix (Main Deco reboot) do fit this. 

The cause of this may be your mobile devices randomizing their MAC addresses, forcing Deco to allocate multiple IP addresses per each mobile device, during short period of time.

There are different ways to fix that, but to verify if this is really the only issue with your Deco mesh, see this FAQ: How to change LAN IP address of Deco? Follow it, but instead of changing LAN IP address - change only Subnet mask. I am guessing it is currently 255.255.255.0 - then, choose 255.255.0.0 and see if it helps. That will significantly increase number of IP addresses Deco can allocate to devices, and if all problems go away after that - you can research other fixes for "MAC randomization," or just keep that one.

  @Alexandre. Thanks, have made change you suggested, will let you know if this helps.

If it is down to exhausting IPs through Mac randomisation, just surprised I didn't notice this before. It could be coincidence but it only seems to have been problematic since the 1.5.6 upgrade.

  @Zarquon 

I hope that change works.

It is possible that just around Deco firmware 1.5.6 upgrade you also got iPhones upgrades that make iPhone treat each WiFi node as an individual network. The point of MAC randomization is to prevent smartphone tracking. When smartphone moves through huge WiFi mesh (such as large shopping mall) and if it does not change MAC when switching between WiFi mesh nodes, iPhone owner movements are quite trackable through the whole area. Map MAC address to person's face image taken by security camera at the entry to the shopping mall, and you will know exactly who that person is and how that person moved through the mall. 

Alternatively, something in Deco firmware 1.5.6 may have forced iPhone treat different Deco mesh nodes as independent and individual networks, thus forcing randomization every time iPhone moves between WiFi nodes.

Whatever trigger is, if MAC address randomization is root cause of issues you described, it would be easy to find a fix or workaround. For example, at my house, for every new device I allow to join my Deco Main Network, I disable MAC randomization in that device for my network SSID.