Channel 15 looks healthy enough. You can expect that higher number (75) due to the zigbee devices currently using that channel, though the fact that 1 channel above (16) reports a much higher usage of 78 than a channel below (14, at 33) might be a bit concerning.
Are you absolutely sure nothing else has changed in your environment in the past week? Did you maybe enable IPv6 routing on your router or HA (your error here seems to indicate it’s using IPv6, which is only typically needed for Matter)?
If you’re not aware of any changes and a wifi scanner doesn’t reveal anything suspicious, then it might be time to move all your zigbee devices to a different channel. 11 looks pretty clear, but 25 is a close second place and is safer with certain brands like Aqara. Note you might need to change your wifi channel back to 1 if you use channel 25 - this might not be needed if you stick to zigbee channel 11
Maybe, if it’s spamming the network and flooding it. Usually only happens with misbehaving power-reporting plugs or mmwave presence sensors. Leave it unplugged for a week and monitor your network stability - if it improves, you’ve found your culprit.
The problem turned out to be caused by an outdoor lamp. And not because of the Philips Hue bulb that was in it, but because of the fixture.
No idea how that is possible, but there was a WiZ bulb in it before and that also caused problems then. That bulb also dropped out of the WiFi network every time.
And now with a Zigbee bulb, the entire Zigbee network was disrupted.
Since we replaced that Hue bulb with a normal “dumb” bulb, our Zigbee network is stable again.
Zigbee bulbs unless otherwise engineered (sengled does this) repeat for the network.
When a zigbee repeater drops it interrupts network comms because they stop propagating messages. So if you had a critical router that was kinda working… (in a light fixture that’s blocking signal) and drops…
If you must have a controllable bulb there try a sengled. It won’t act as a repeater so even if it has trouble itself it won’t kill the network.
If you really need to have that fixture automated, then get a zigbee relay module and stick it behind the switch inside the house.
Or just follow @NathanCu’s advice and get a Sengled (end-device) bulb. Note that it might still go offline periodically due to the fixture, but at least it won’t take down your network unless it spams it with reconnection messages.