I looked at the piuparts log:
https://bugs.debian.org/cgi-bin/bugreport.cgi?att=1;bug=825121;filename=sipdialer_1%3A1.10.2-1.log.gz;msg=5 and it confirms that the freeradius-client package was installed before the radcli package, therefore, the preinst script tries to migrate the configs. This is expected behavior. I've downgraded the severity in case anybody wants to discuss it further, otherwise this bug can potentially be closed.
