I'm having the same problem and I have relogged. Several times actually, over more than 24 hours.
I was trying to send no copy items so I thought maybe that's what the problem was, but no. I tried sending a full perm item to marketplace and had the same problem.
So now, what do you have for me?
The brilliance of "Have you tried turning it off and back on again?" has been found wanting in this case because yes, I have tried that and it did no good.