We have a corporate account (EDUCAUSE JIRA) and we purchased the additional UI Mockup service. We can make mockups, but cannot attach the mockups to tickets via the save mechanism. Attached is a screenshot of the error message, shown in the black box, bottom left.

How to replicate the problem:
1) We open an issue/ticket.
2) In the "More" drop-down list, we select "Add/Edit UI Mockup"
3) Create a mockup, does not matter if it is simple or complex.
4) Click the "Save" button in the Mockup menu bar

Thanks for looking into this.
-Alex