Please please please do not take what I say as 100% as I am only just getting into this myself.
Make sure you follow up and confirm yourself before making any changes.
Briefly from what I have seen around the forums, it looks like you have to have nether=true in the server.properties file
I had the same kind of problem with nether portals, I would get the purple wavey screen and the portal transfer sound, but then nothing loading.
Then I discovered I had nether=false.
Partly where I got my info
http://forum.feed-the-beast.com/thr...ight-forest-worlds-will-load.2418/#post-24141
Because of the problems with the nether portal, I did a twilight forest portal and jumped through it. I subsequently got stuff in just black nothingness, so I'm looking for a solution to that problem if anyone has one.
EDIT: Added the link