Ahh, this often happens on servers and frequently on my own, I am afraid to do this you will either have to go to the files and delete your player.dat folder on your world, or delete the world save if you don't want to uninstall and re download the mod, on servers this can be fixed similarly by deleting the players.dat folder on the particular world.
Well this is strange, I unfortunately cant tell you anything unless you have creative mode on :P if you dont, it could either be a problem with the computer or the mod itself, restart your computer, and if the problem persists, redownload the mod, thank you.
I have experienced that issue too, the invisible area one, but it does not go away... I could use a little help in the understanding of this
I have to agree with marcosmapf's first post on this thread, We need more nether related items/mobs/possible NPC's.