I installed recently Slackware 15 in my PC and haved the same problem "closed connection from ip".
I need to said i haved a preview build of rathena in slackware 14.2., so, I need rebuild the server in Slackware 15.
Rebuild Rathena Lunix.
make clean
./configure --enable-packetver=YYYYMMDD ( your client version, example 20151029)
make.
Then I can connect with my client without problems.