Not sure if this helps or not, but for what it's worth ...
I was also having problems with links not being submitted. Kept saying that the captcha form was answer was blank or incorrect, even though I tried several variations of both the visual and custom ones on and off. Bizarrely, this was happening on the second library I'd created, while Library 1 worked fine. As far as I was aware the second library was an exact copy of the first, except that the wording on the submission form for that one was different. I created a Library 3, copying 1 again, and this works fine for submissions. (Haven't yet changed the form wording ... maybe that'll break things again!) Have given up trying to figure out why ... just happy that it now works!
Now if only I could get them to generate a confirmation email on link submission ... my reason for finding this thread! Still, if I can't crack this one by playing, I'll start a new and relevant thread.