Very strange problem that has me tearing my hair out. I've got a mesh network consisting of 3 XE75 nodes, connected by an ethernet backbone and accessing internet via a Virgin modem (I'm in the UK). This has been working without problems for well over a year, and I've not made any changes to configs for some time.
 

Last week, two different machines connected to the network (one on a wired connection, one on wifi, both running Windows 11 and google chrome, all up to date with updates) started behaving unusually. Two websites I use regularly, bbc dot co dot uk and reddit dot com completely refused to load on my chrome browser (everything else is working normally). I get one of two errors:
 

This site can’t be reached

XXX unexpectedly closed the connection.

or

This site can’t provide a secure connection

XXX sent an invalid response.

Try running Windows Network Diagnostics.

ERR_SSL_PROTOCOL_ERROR

I get the same error on MS Edge browser, but I can access these sites normally on firefox. On another laptop with the same Windows 11, chrome setup, same user account everything works perfectly normally.

If I change network by connecting via my phone's hotspot, or using my work wifi, then everything works normally. So, it's something related to my home network.

On my laptop, in frustration I've done a completely clean reinstall of windows, and installed only chrome. Disabled all extensions, run in incognito mode, disabled QUIC, checked date/time is correct, disabled all virus checker and firewalls and still get exactly the same behaviour.

On my Deco I've disabled all the security features - Web protection, Intrusion protection, IoT Protection, Parental controls, but still the same problem. Rebooted all Deco units multiple times.All units are up to date on updates.

Nothing obvious showing up on logs, seaching for the IP address of the affected machines

The only thing I've not done yet is a full factory reset of the Decos - that will take a little time to set everything back up again, and won't be popular with the rest of the house so will have to wait.

Appreciate any suggestions for anything else I could try or any ideas of where the problem may lie.