New - RBR850 / RBS850 Firmware Version v4.6.7.5 Released

Have an RBK853, usually setup in a daisy chain configuration due to the house shape being an extended L.  RBR is Ethernet connected to the fibre router, and is configured as a router with double-NAT (hasn't caused any problems).  RBSs are WiFi connected.

Was running the old production release (4.6.3.16?) until last weekend.  It had been pretty stable in the past but the network connections were becoming intermittent so decided to upgrade to 4.6.7.5.

Installed it manually to the RBSs, rebooted them with a good 5 minutes delay, then the RBR, again with a reboot.  Then for good measure rebooted the lot, started with the fibre router, RBR, then RBSs.  This is how they're normally recycled and it usually works well.  It did in this case too, and the installation was completed successfully.

The household is mostly Macs and iPhones.  Some SONOS, smart TVs, etc.

My wife has a Chromebook for work.  Whenever she starts it up, after about 10-15 seconds, both RBSs flash white, and stop working.  The RBR is fine, and she can move to a different room, closer to it, and continue working OK.  The only solution seems to be to recycle the setup again.  But the problem is reproducible, and it happens every time.

Does anyone else have this problem with 4.6.7.5?

Or experienced it with the previous betas?  It wasn't a problem with the production release.

Any ideas how to investigate and resolve this (I'm out of support)?