After Update 3.0.4
Directories were found in your Piwik but we didn't expect them. --> Please delete these directories to prevent errors. <-- Directory to delete: plugins/Morpheus/icons/submodules
That's really odd. That directory should have never been there. You can safely delete it. Did you by change use the package from git(hub)?
I used https://builds.piwik.org/piwik-3.0.4.zip to install piwik.
Hm. That directory does exist in the zip file. Guess we need to delete it in the build script
Ah, I created a new folder to make the repo more organized and didn't notice you changed the build-script in https://github.com/piwik/piwik-package/commit/e26d2ab6ccc088de72087d429056b1f0b84c537e to only delete the list of existing files.
Proposed a PR to fix it