Just remembered something: Which tail executable are you using?
If I recall correctly the /usr/xpg4/bin/tail version has the same options as the linux version.
If that is the case then the following should work:
/usr/xpg4/bin/tail -n+20 server.log
And in case that doesn't work, you could use sed to produce the same result:
sed -n '5,$p' server.log