I do custom builds, incl. uhttpd, but without LuCI. Using uhttpd for https-connections does not work any more, unless in my case, libuhttpd-openssl explixitly is included into image.
In the past, there was an option in "make menuconfig" , when selecting uhttpd, to include ssl-support, too. This option is gone. So, best would be to re-introduce explicit selection of ssl. Or to include a reasonable ssl-support library, as default, and to include pxg5, too, may be.
Alternatively you can use nginx instead, works just fine with and without SSL.