Problems modifying cat/articles in Admin

[peterf]

I'm facing problems modifying any categories/articles from my Admin. I can no longer add/delete/modify a category or product.
Article: I get the edit article page, but as soon I press the show button I get to the main Admin page (only the menu row).
Other items like Admin-Tools-PageEditor etc works fine.
I can't figure out where I went wrong or what. Please who can help 'cause my live shop is stuck now.

[peterf]

Thanks to Dr. Byte with one of his posts!!

Somehow one of my files in my Admin, in my case is was the file preview_info.php in the folder admin-includes-module-product, was not ftp-ed correctly.

I checked all my admin files and figured out that this particular files had zero bytes. After uploading this file again the problem was solved!