Be sure that no other freeradius is running and also that you have enough rights to open such a port.

Look in your inet.d or similar to avoid that another service is run instead of the planned freeradius.
 
Thanks for the quick answer. I have thought the same because also some old mailing list post seemed to be related to this problem.
I checked this possible problem before posting, but as far as I can see there is no other instance of freeradius running (ps -e | grep freeradius returns empty), and nothing is listening on that port (according to netstat). I also tried to change port several times but it's not working....
In /etc/services the port 1812 both tcp and udp are correctly assigned to radius (in fact in the error message it correctly use the port 1812).
 
Regards,
 
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Stefano D'Avella