I already thought of this idea, Iliya :)
Needed file has "readable for all" permissions:

[root@motp-a ~]# ls -l /var/www/html/radius-client.php
-r--r--r--. 1 radiusd root 337 Jul 19 21:43 /var/www/html/radius-client.php

Also, I've just checked - you are right. Radius server is running under radiusd user:

[root@motp-a ~]# ps aux | grep radius
radiusdšš 4179š 0.0š 0.5š 60444š 2748 ?ššššššš Sslš 16:14šš 0:00 /usr/sbin/radiusd -d /etc/raddb
rootššššš 4187š 0.0š 0.1šš 4336šš 752 pts/0ššš S+šš 16:15šš 0:00 grep radius

But owning all needed files by this user and runing in debug mode under him give no result or additional info. Radius still works with -X flag and fails in service mode with same error.

And, if this has any value, all this running on CentOS release 6.3, not Ubuntu. Sorry for confusing paste site, it's just comfortable place for sharing long files.