I just upgraded my Linksys WRT1900ACS to 21.02.0 and am attempting to add a line to /etc/rc.local as suggested here: WRT1900ACS v1 hangs frequently on OpenWrt 21.02.0-rc3. Every time I click 'Save' I get 'Unable to save contents: XHR request aborted by browser'. After that OpenWRT can no longer find LuCi - attempting to navigate to any page returns a 404.
I guess I can SSH in to make the changes to /etc/rc.local, but would very much like to understand why I can't make those changes through LuCi. Also, losing the GUI sounds like a bug to me, or at least something that should be documented. I am running Firefox; I haven't yet tried this with Chrome.