It sounds as if some permissions might be wrong in your Piwik installation.
Check these:
Directory "config" and all its contents: universal read/write/execute initially. When Piwik is configured and running, you can set this back to read-only.
Directory "tmp" and all its contents: universal read/write/execute. You may not be able to set permissions on some of this directory's contents because of "ownership" settings. Don't worry about that.
Note:
Some servers require permissions for PHP scripts to be set to 755 instead of 644. MIne is like that, so I set everything to 755 except where 777 is needed.
Check these:
Directory "config" and all its contents: universal read/write/execute initially. When Piwik is configured and running, you can set this back to read-only.
Directory "tmp" and all its contents: universal read/write/execute. You may not be able to set permissions on some of this directory's contents because of "ownership" settings. Don't worry about that.
Note:
Some servers require permissions for PHP scripts to be set to 755 instead of 644. MIne is like that, so I set everything to 755 except where 777 is needed.