This message appeared when I clicked to upgrade automatically:
Incompatible archive: PCLZIP_ERR_BAD_FORMAT (-10) : Unable to find End of Central Dir Record signature
Piwik One Click Update has been cancelled. If you can’t fix the above error message, it is recommended that you manually update Piwik.
Please check out the Update documentation to get started!
UPDATE: I’ve since manually upgraded and that worked fine.