I would guess, if this isnt relevant to the
setenv.set-environment = ( "PATH" => "/usr/bin:/usr/sbin:/bin:/sbin" )
which was added to lighttpd config since 3.6. And it doesnt work correctly, according to lighttpd log.
Anyway, installing packages through opkg (that means also Luci) isnt good idea. Much better is use updaters user.lua (/etc/updater/user.lua), in your case
Install "collectd-mod-thermal"
and then run /usr/bin/updater.sh
That way updater will be aware of your custom packages, and you dont loose them after each update. That should be mentioned on the official wiki page. Turris team are aware of problem opkg raw use VS updater. And since they need force users to use updater (far less problems through updates), I think this isnt emphatised enough.
Simple custom field or basic UI in foris, which would edit user.lua will be solution for those, who doesnt know opkg/ssh etc. I mean user would be able to write there custom modules, which want install and preserve during updates. There are groups of their packages (nas, majordomo), which is fine, but there are far many more plugins usefull for end users.
Also I recommend add those lines into /etc/config/updater. You will have opportunity to check new configs, delivered by updater against your and decide, if change is welcomed or not.
config override 'override'
option disable '1'
P.S.: thermal package doesnt work within luci - doesnt show graphs.