The button worked. When Assets cannot pull the file down in the background it opens the picture in a new tab instead, so that you can still save it. It does that silently, with no message, which is why it reads as a broken Download.
Saving it from the tab
Right click the image and choose Save image as. What you get is the same full resolution file the button would have handed you. On a phone or tablet, press and hold the image instead.
Why it happens
Download normally fetches the whole file first and gives your browser a finished copy, named uni-render- followed by the job's identifier. That fetch has to be allowed by the storage the image sits on. When it is refused, the code opens the picture directly rather than showing an error, on the reasoning that opening it always works and an error would leave you with nothing at all.
None of this involves your account, your membership or your credits. The same asset can download cleanly in one browser and open a tab in another.
If no tab opens either
Your browser blocked the pop-up. Allow pop-ups for uni.xyz and press Download again, or take the other route: open the job on the Jobs page and use Download Full Resolution there, which is the same file.
The same picture lives in two places
Every completed image appears on both the Assets page and the Jobs page. If one route gives you trouble, try the other before assuming the file is damaged.