Just thought I'd pop in and let anyone who may be dealing with this irritating problem know how to get through it.
I google'd this issue earlier this evening and was met with a couple posts on the subject, both of which had our esteemed coder, LexManos, reacting in a rather douchey manner instead of actually working the situation out or trying to help. No disrespect intended, but here;
"The issue is that you are failing at install ALL the files that forge ships, this isn't a forge issue, you just need to be better at copy/pasting files -.-
There is no help we can give you beyond, do it properly."
It appears that all of the .class files don't like being copied with Ubuntu (10.04, although I believe the issue is the same with 12) and it's default archiving software. All you have to do is go through the folders and identify the ones that the class files don't properly copy into, then simply re-copy the missing class files, but not the folders, for some reason they won't copy if you attempt this, and your problem is solved.
Just finished setting up my personal server on my old slave machine now, works like a dream.
TL;DR Double check all the .class files, like LexManos said, although don't think it's your fault. It's not, it's just the os being wonky.