Ok I finally got a message back from the helpdesk today regarding the problems I've been having with submissions. It turns out that the cause of all the issues is with the Ad Muncher program that I have been using. I turned off all the filters for DeviantART and the submission worked perfectly.

So, if you use Ad Muncher and you've had the same issues as me, try filtering DeviantART out.

I'm so relieved. It would have been quite sad to leave here. Now I don't have to