OpenWRT is an open source wifi router operating system. I run it on my wifi router, in place of the manufacturer’s firmware/operating system.
If you upgrade OpenWRT, you may find that the web interface (LUCI) stops working. This may be because upgrading with a binary image bundle, as recommended, may not update packages that were manually installed using opkg. In particular, the SSL feature for the LUCI may break.
To fix it, I had to reinstall libuhttpd-mbedtls:
opkg install --force-reinstall libuhttpd-mbedtls