Does it give a reason for why? It may be that you just will have to wait another day or two before the site is working properly again. I had that problem once before where for some inexplicable reason i couldn't upload any documents.
FFN (aka The Pit) has been troublesome from the beginning, not just with uploading, too. Those ads--!! Urgh! [Goes off to ffdotnetrants on LJ to rant about it]
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Bunnies aren't cute like everybody supposes! They've got them hoppy legs and twitchy little noses! And what's with all the carrots! Why do they need such good eyesight for anyway?! Bunnies! Bunnies! It must beBUNNIES!!
It'd be a bit of a departure from the familiar, but might I suggest (at least as a mirror) www.furrag.com? Most of what's there is original furry fiction, but there are a couple of fanfics there (even an LU one), so you wouldn't be out of place. The site has never had any reliabilty problems to date, and the operators actually give a rip about site design and aesthetic layout.
And there are NO ADS! NONE! ZIP! ZERO!
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Ah, Tech. Tu chauffes mon cœur comme un jour en été…
Fanfiction always have these problems, the reason behind the inability of uploading is due to a few reasons. One reason is the document manager is under unofficial maintenance or something like that. Should be up in a few hours.
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"If you're reading this, you're feeding my ego." ~me
You might have to wait up to a week or two. I had that problem where i couldn't upload my documents for a long while. But then it just suddenly worked after that time.