Hi , Got exactly same problem.
When active connection is high (Above 2-3k) the following command
ubus call luci getRealtimeStats '{ "mode": "conntrack" }'
Works just fine and returns a reply.
And this
ubus call luci getConntrackList
Just hangs an causes Luci to return
/usr/lib/lua/luci/dispatcher.lua:426: /etc/config/luci seems to be corrupt, unable to find section 'main'
Only restarting rpcd gets it back using
service rpcd restart
My device has plenty of ram (RPI) so i think the code for returning the connection list doesnt really work with large numbers, I guess it should only return the top 100 connections or something like that.
Using my mobile app on my router (https://github.com/hagaygo/OpenWRTManager) with "Active Connections" overview causes LuCI to keep crashing while there is a lot of active connections.
It uses same API as real time page.