I too got an error, when I submited A3 yesterday. Then I thought of resubmiting today, but the link has changed to "closed" - has it been printed? Last time this happened I called the UNISA contact center, they told me that according to their screen, they have recieved it. I explained the problem to them and they transfered me to assignments, who then transfered me to the call centre again. I think there is a bug in the creation of the confirmation page (that's why they need to do maintainence on the 17 July).
Remeber after you have selected the file and type, it uploads it to Unisa, then it gives you a final choice and once you press OK it is sent.
Its best just to leave it, if myunisa shows it registered then it sould be there.