While trying to setup syslog-ng on Solaris 10, I've come across:

# /usr/local/sbin/syslog-ng
Error parsing command line arguments: Conversion from character set '646'
to 'UTF-8' is not supported

Several places on the net lead me to suspect a missing charset.alias file. And indeed, syslog-ng was looking for one:

# truss -elfda /usr/local/sbin/syslog-ng 2>&1 | grep charset
29850/1:  1.9301 open64("/usr/local/lib/charset.alias", O_RDONLY) Err#2 ENOENT

# locate charset.alias
/usr/lib/charset.alias
/usr/lib/sparcv9/charset.alias
# ln -s /usr/lib/charset.alias /usr/local/lib/charset.alias

...Now syslog-ng is working fine. This does not seem to be syslog-ng related, but may help other applications as well. To be confirmed though....