Your keystore file worked for my project.
So I can only assume it is something to do with your project files.
Final things to check:
Are you certain it is a ZIP file?
Read the file in a text/hex editor and look at the header, the first 2 letters should be PK.
Are your files in the ZIP file contained in a directory named "media"?
Not sure if this is the problem, but that's the way my zip file is.
Last, maybe there is some miscommunication between your machine and the server.
I'm attaching the media.zip file I just used to make an APK. I used your keystore.
Give it a try on your end. If it doesn't work then I have to assume there is something between your machine and the server causing the issue.
Here's a screenshot of what I put into the form