Having struggled with this for months on and off and I finally identified the problems I’ve been having with my Cpanel/Ubuntu install of Piwik.
The installation instructions do not cover the issues you’ll hit if your *nix distro is not inheriting folder permissions when it creates new cached templates in template_c as well as other tmp folders/files.
The solution is to use the setfacl command to set permission defaults for your specific tmp folder.
setfacl -Rm d:u::rwx,d:g::rwx,d:o::rx /home/[b]piwikhost[/b]/public_html/[b]piwik[/b]/tmp
You will probably have to chown and chmod recursively to resolve any folders that were created before you ran the above but this seems to fix things