KiwiBuilder03 on "500 (Internal Server) Error when uploading new theme files"

I just bought a theme from Theme Forest (Modernist) and downloaded the installation files to my computer.
When I go to upload the files from my computer under "Install Themes", I get a "500 (Internal Server) Error" with the additional message "we've happened onto abit of a problem/ Beware sharks. ...that we're working furiously to correct. Things will be up and running again soon. Thanks for your patience."
I have tried multiple times last night and a couple of times again this morning but still the same message.
Any idea what could be going wrong?
Thanks in advance.