Hi
On sandbox, I get this error trying to create a new item:

Error
You are not allowed to access this page. You can only use your personal components.

I don't get this on my own installation. It's just a plain table. Attached screenshot of the table that is creating the error.
Btw: Can't use png or jpg for attachments...

See the table here: www.dropbox.com/s/mw704rdeborpyvp/error.jpg

Deleted the Joomla user fields and recreated them - works now