I don't know if this helps, but I think I have been experiencing this kind of problem. I use a Mac, and have the latest version of java so far. I am not the one to know the antivirus program that I am currently using, but I am 97% sure that has nothing to do with this problem. when I run the java, a "start" program runs. Then a big "technic" displays with a bar under it. the bar fills, and then nothing happens. The start program closes.
However, this doesn't always happen. every once in a while this doesn't happen and it works just fine.
I have checked and double-checked for a fix to this. I really would like to play my modpacks. suggestions?