Troubleshooting

/usr/lib/lua/luci/template.lua:55: Failed to execute template 'sysauth|view'.

After a firmware update it may happen that when trying to enter Luci the following error appears

/usr/lib/lua/luci/template.lua:55: Failed to execute template 'sysauth'.
A runtime error occurred: /usr/lib/lua/luci/template.lua:55: Failed to execute template 'header'.
A runtime error occurred: /usr/lib/lua/luci/template.lua:55: Failed to execute template 'themes/mcat/header'.
A runtime error occurred: [string "/usr/lib/lua/luci/view/themes/mcat/header.h..."]:23: attempt to call field 'node_childs' (a nil value)
stack traceback:
	[string "/usr/lib/lua/luci/view/themes/mcat/header.h..."]:23: in main chunk
stack traceback:
	[C]: in function 'n'
	/usr/lib/lua/luci/template.lua:55: in function 'render'
	/usr/lib/lua/luci/dispatcher.lua:592: in function 'include'
	[string "/usr/lib/lua/luci/view/header.htm"]:3: in main chunk
stack traceback:
	[C]: in function 'n'
	/usr/lib/lua/luci/template.lua:55: in function 'render'
	/usr/lib/lua/luci/dispatcher.lua:592: in function 'include'
	[string "/usr/lib/lua/luci/view/sysauth.htm"]:1: in main chunk
stack traceback:
	[C]: in function 'n'
	/usr/lib/lua/luci/template.lua:55: in function </usr/lib/lua/luci/template.lua:47>
	(tail call): ?
	(tail call): ?
	/usr/lib/lua/luci/dispatcher.lua:416: in function </usr/lib/lua/luci/dispatcher.lua:415>

Something is wrong with the theme. SSH to your router and change default theme to bootstrap. From CLI:

root@OpenWrt:/etc/config# uci set luci.main.mediaurlbase=/luci-static/bootstrasp
root@OpenWrt:/etc/config# uci commit luci

If it returns a "parse error" edit manually luci config file (/etc/config/luci) with vi or nano and change "option mediaurlbase" with the correct bootstrap path you will see in the config internal 'themes' section:

config core 'main'
        option resourcebase '/luci-static/resources'
        option ubuspath '/ubus/'
        option lang 'en'
        option mediaurlbase '/luci-static/material'

config extern 'flash_keep'
        option uci '/etc/config/'
        option dropbear '/etc/dropbear/'
        option openvpn '/etc/openvpn/'
        option passwd '/etc/passwd'
        option opkg '/etc/opkg.conf'
        option firewall '/etc/firewall.user'
        option uploads '/lib/uci/upload/'

config internal 'languages'
        option en 'English'

config internal 'sauth'
        option sessionpath '/tmp/luci-sessions'
        option sessiontime '3600'

config internal 'ccache'
        option enable '1'

config internal 'themes'
        option Argon '/luci-static/argon'
        option Bootstrap '/luci-static/bootstrap'
        option Material '/luci-static/material'
        option OpenWrt2020 '/luci-static/openwrt2020'
        option OpenTomato '/luci-static/opentomato'
        option ATMaterial '/luci-static/atmaterial'
        option ATMaterial_red '/luci-static/atmaterial_red'
        option ATMaterial_Brown '/luci-static/atmaterial_Brown'
        option BootstrapDark '/luci-static/bootstrap-dark'
        option BootstrapLight '/luci-static/bootstrap-light'
        option tomato '/luci-static/tomato'
        option Argonne '/luci-static/argonne'
        option mcat '/luci-static/mcat'
        option chuqitopd '/luci-static/chuqitopd'