I can't delete any uploaded files from file-field

There are two times when this message appears (on a usual basis).

1. A user edits a node, saves the changes, uses the back button to navigate back, makes additional changes, and saves.
2.Two users attempt to edit the same node at the same time. The second user to click save will receive this error.

Try flushing your cache and closing your session.

I had this exact same behaviour consistently on some urls of my site without it being related to any of the cases mentioned in #1.
The cause in my site had to do with the multilingual setup and detection.

What apparently happened was that the language of the current detected interface page ( on the page where the nodeform resided) differed from the current detected language of the ajax call of the filefield (on the same node form). It may be a rare case however, as I have a custom language detection module ( based on url of the page) this may be the case with the standard language by url detection also - I don't know.
I changed interface language detection to use users preferences and not detection by url. This fixed it. No more "already submitted" messages when uploading/removing a file or image.

Thanks @grahamtk! This hint is money! I just stumbled upon this using Media module in a multilanguage site. It wouldn't let me add or remove images/files without saving automatically each time. But setting the interface language detection to "User" solved it.

Cheers, Moritz

@shortspoken: Did you open a bug report on media module? I had the same problem and think changing the language is only a workaround but not a fix. I got this while using the Admin Language Module.

@osopolar: No, you got me wrong. I didn't change the language but the language DETECTION. Thats a huge difference. But I'd consider the Admin Language module as rather buggy and not helpful in a multilanguage site, although its a very useful module if it worked...
Cheers.