Problem:
Custom freeradius configuration lost or overwritten by u-radius package updates. Therefor radius is not starting or working properly anymore.
Investigation:
start radius via
/usr/sbin/freeradius -X
there you might see more info why radius is not starting.
But this tool may also fool you, because, it still says the error, but radius is starting via systemd again.
Solution/ Workaround
Workaround for a customer with a custom config in a file with the same name, after the package update:
cd /etc/freeradius/3.0/sites-enabled
rm default
ln -s /etc/freeradius/3.0/sites-available/custom default
See also Bug 55007