I'm in the process of writing my first package and at first I got this error (package_get_error_is_zero), the important part is in bold,
QuoteAlthough the package was downloaded to the server it appears to be empty. Please check the Packages directory, and the "temp" sub-directory are both writable. If you continue to experience this problem you should try extracting the package on your PC and uploading the extracted files into a subdirectory in your Packages directory and try again. For example, if the package was called shout.tar.gz you should:<br />1) Download the package to your local PC and extract it into files.<br />2) Using an FTP client create a new directory in your "Packages" folder, in this example you may call it "shout".<br />3) Upload all the files from the extracted package to this directory.<br />4) Go back to the package manager browse page and the package will be automatically found by SMF.
Because I had no package to start with, I followed what was written in the error and put my files inside a directory in 'Packages'. Now that I have something working (SMF is saying 'Success'), I put all files back inside a zip or tar.gz (the error stays the same) and maybe I am overlooking something obvious? It is my first package and others do work, I did verify that all files mentioned in the xml also exist in the package.