Start by following this to rule out both interference and lack of routers → Guide for Zigbee interference avoidance and network range/coverage optimization (weird symptoms are usually due to EMF interference from something so before doing anything else start by connecting the Zigbee Coordinator via a very long USB extension cable to get it far way from other devices, and be sure to only connect to to a USB 2.0 port, so connect it via a powered USB 2.0 hub if your computer only have USB 3.x ports. Know that USB 3.x ports and USB 3.x devices will cause serious EMF interference for Zigbee).
If that does not work then also consider upgrading the Zigbee Coordinator firmware to a later version.