i also had this issue, i sent in a ticket and this was the response
"I am sorry that you are having difficulty with the Merchant Outbox and uploading content to the Second Life Marketplace.
Please download the Merchant Outbox Project Viewer, located here:
http://wiki.secondlife.com/wiki/Release_Notes/Second_Life_Project_Merchant_Outbox/3.6.13.284731
If the Merchant Outbox Project Viewer does not correct the issue, please create a Second Life Jira Bug Report with details regarding the issue that you are having and include any messages that are displayed in the viewer when you attempt to add items to the Merchant Outbox."
that worked for me, after i did this, i just started doing it the regualr way and it started working again.